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<svg class="nj-fig nj-charts nj-charts--compile" viewBox="0 0 470 516" role="img" aria-labelledby="nj-charts-ct nj-charts-ct-d" xmlns="http://www.w3.org/2000/svg">
|
||||
<svg class="nj-fig nj-charts nj-charts--compile" viewBox="0 0 470 620" role="img" aria-labelledby="nj-charts-ct nj-charts-ct-d" xmlns="http://www.w3.org/2000/svg">
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<title id="nj-charts-ct">Change in compile time against the default configuration at -O0, Apple clang and G​CC 16</title>
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||||
<desc id="nj-charts-ct-d">Grouped horizontal bars, negative is faster. json_fwd.hpp instead of json.hpp on a header-only translation unit: Apple clang -62 %, G​CC 16 -82 %; extern template on a parsing translation unit: Apple clang -7 %, G​CC 16 -21 %; JSON_NO_AUTOMATIC_UDLS on a model translation unit: Apple clang -11 %, G​CC 16 -11 %; JSON_NO_AUTOMATIC_UDLS on a parsing translation unit: Apple clang -1 %, G​CC 16 -2 %; JSON_HAS_FILESYSTEM=0 + JSON_NO_IO on a model translation unit: Apple clang -8 %, G​CC 16 -11 %; JSON_HAS_FILESYSTEM=0 on a model translation unit: Apple clang -5 %, G​CC 16 -9 %; JSON_NO_IO on a model translation unit: Apple clang -2 %, G​CC 16 -2 %; JSON_USE_GLOBAL_UDLS=0 on a model translation unit: Apple clang 0 %, G​CC 16 -1 %.</desc>
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||||
<desc id="nj-charts-ct-d">Grouped horizontal bars, negative is faster. json_fwd.hpp instead of json.hpp on a header-only translation unit: Apple clang -62 %, G​CC 16 -82 %; extern template on a parsing translation unit: Apple clang -7 %, G​CC 16 -21 %; JSON_NO_AUTOMATIC_UDLS on a model translation unit: Apple clang -11 %, G​CC 16 -11 %; JSON_NO_AUTOMATIC_UDLS on a parsing translation unit: Apple clang -1 %, G​CC 16 -2 %; JSON_HAS_FILESYSTEM=0 + JSON_NO_IO on a model translation unit: Apple clang -8 %, G​CC 16 -11 %; JSON_HAS_FILESYSTEM=0 on a model translation unit: Apple clang -5 %, G​CC 16 -9 %; JSON_HAS_STD_FORMAT=0 on a model translation unit: Apple clang -1 %, G​CC 16 -4 %; JSON_HAS_RANGES=0 on a model translation unit: Apple clang -4 %, G​CC 16 -1 %; JSON_NO_IO on a model translation unit: Apple clang -2 %, G​CC 16 -2 %; JSON_USE_GLOBAL_UDLS=0 on a model translation unit: Apple clang 0 %, G​CC 16 -1 %.</desc>
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<rect class="nj-charts-clang" x="8" y="10" width="12" height="12" rx="3"/>
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||||
<text class="nj-charts-leg" x="26" y="20.5">Apple clang</text>
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<rect class="nj-charts-gcc" x="126" y="10" width="12" height="12" rx="3"/>
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||||
<text class="nj-charts-leg" x="144" y="20.5">G​CC 16</text>
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<line class="nj-charts-grid" x1="47.4" y1="58" x2="47.4" y2="472"/>
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||||
<text class="nj-charts-tick" x="47.4" y="488">−80 %</text>
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||||
<line class="nj-charts-grid" x1="126.2" y1="58" x2="126.2" y2="472"/>
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||||
<text class="nj-charts-tick" x="126.2" y="488">−60 %</text>
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<line class="nj-charts-grid" x1="205" y1="58" x2="205" y2="472"/>
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<text class="nj-charts-tick" x="205" y="488">−40 %</text>
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||||
<line class="nj-charts-grid" x1="283.8" y1="58" x2="283.8" y2="472"/>
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||||
<text class="nj-charts-tick" x="283.8" y="488">−20 %</text>
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||||
<text class="nj-charts-tick" x="362.6" y="488">0 %</text>
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||||
<text class="nj-charts-tick nj-charts-tick--l" x="8" y="506">← faster</text>
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||||
<text class="nj-charts-tick nj-charts-tick--r" x="462" y="506">slower →</text>
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<line class="nj-charts-grid" x1="47.4" y1="58" x2="47.4" y2="576"/>
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<text class="nj-charts-tick" x="47.4" y="592">−80 %</text>
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||||
<line class="nj-charts-grid" x1="126.2" y1="58" x2="126.2" y2="576"/>
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<text class="nj-charts-tick" x="126.2" y="592">−60 %</text>
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||||
<line class="nj-charts-grid" x1="205" y1="58" x2="205" y2="576"/>
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||||
<text class="nj-charts-tick" x="205" y="592">−40 %</text>
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||||
<line class="nj-charts-grid" x1="283.8" y1="58" x2="283.8" y2="576"/>
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<text class="nj-charts-tick" x="283.8" y="592">−20 %</text>
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||||
<text class="nj-charts-tick" x="362.6" y="592">0 %</text>
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||||
<text class="nj-charts-tick nj-charts-tick--l" x="8" y="610">← faster</text>
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<text class="nj-charts-tick nj-charts-tick--r" x="462" y="610">slower →</text>
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<text class="nj-charts-rowlabel" x="8" y="62"><tspan class="nj-charts-code">json_fwd.hpp instead of json.hpp</tspan><tspan class="nj-charts-sub"> · header-only TU</tspan></text>
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<path class="nj-charts-clang" d="M362.6 70 H122.32 Q118.32 70 118.32 74 V78 Q118.32 82 122.32 82 H362.6 Z"><title>json_fwd.hpp instead of json.hpp, header-only TU (Apple clang): −62 %</title></path>
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<text class="nj-charts-val" x="367.6" y="80.5">−62 %</text>
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@@ -46,15 +46,25 @@
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<text class="nj-charts-val" x="367.6" y="340.5">−5 %</text>
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<path class="nj-charts-gcc" d="M362.6 344 H331.14 Q327.14 344 327.14 348 V352 Q327.14 356 331.14 356 H362.6 Z"><title>JSON_HAS_FILESYSTEM=0, model TU (G​CC 16): −9 %</title></path>
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<text class="nj-charts-val" x="367.6" y="354.5">−9 %</text>
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<text class="nj-charts-rowlabel" x="8" y="374"><tspan class="nj-charts-code">JSON_NO_IO</tspan><tspan class="nj-charts-sub"> · model TU*</tspan></text>
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<path class="nj-charts-clang" d="M362.6 382 H358.72 Q354.72 382 354.72 386 V390 Q354.72 394 358.72 394 H362.6 Z"><title>JSON_NO_IO, model TU (Apple clang): −2 %</title></path>
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<text class="nj-charts-val" x="367.6" y="392.5">−2 %</text>
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<path class="nj-charts-gcc" d="M362.6 396 H358.72 Q354.72 396 354.72 400 V404 Q354.72 408 358.72 408 H362.6 Z"><title>JSON_NO_IO, model TU (G​CC 16): −2 %</title></path>
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||||
<text class="nj-charts-val" x="367.6" y="406.5">−2 %</text>
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<text class="nj-charts-rowlabel" x="8" y="426"><tspan class="nj-charts-code">JSON_USE_GLOBAL_UDLS=0</tspan><tspan class="nj-charts-sub"> · model TU*</tspan></text>
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<path class="nj-charts-clang" d="M362.6 434 h0.5 v12 h-0.5 Z"><title>JSON_USE_GLOBAL_UDLS=0, model TU (Apple clang): 0 %</title></path>
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<text class="nj-charts-val" x="367.6" y="444.5">0 %</text>
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||||
<path class="nj-charts-gcc" d="M362.6 448 H362.6 Q358.66 448 358.66 451.94 V456.06 Q358.66 460 362.6 460 H362.6 Z"><title>JSON_USE_GLOBAL_UDLS=0, model TU (G​CC 16): −1 %</title></path>
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<text class="nj-charts-rowlabel" x="8" y="374"><tspan class="nj-charts-code">JSON_HAS_STD_FORMAT=0</tspan><tspan class="nj-charts-sub"> · model TU</tspan></text>
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||||
<path class="nj-charts-clang" d="M362.6 382 H362.6 Q358.66 382 358.66 385.94 V390.06 Q358.66 394 362.6 394 H362.6 Z"><title>JSON_HAS_STD_FORMAT=0, model TU (Apple clang): −1 %</title></path>
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||||
<text class="nj-charts-val" x="367.6" y="392.5">−1 %</text>
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||||
<path class="nj-charts-gcc" d="M362.6 396 H350.84 Q346.84 396 346.84 400 V404 Q346.84 408 350.84 408 H362.6 Z"><title>JSON_HAS_STD_FORMAT=0, model TU (G​CC 16): −4 %</title></path>
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||||
<text class="nj-charts-val" x="367.6" y="406.5">−4 %</text>
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||||
<text class="nj-charts-rowlabel" x="8" y="426"><tspan class="nj-charts-code">JSON_HAS_RANGES=0</tspan><tspan class="nj-charts-sub"> · model TU</tspan></text>
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||||
<path class="nj-charts-clang" d="M362.6 434 H350.84 Q346.84 434 346.84 438 V442 Q346.84 446 350.84 446 H362.6 Z"><title>JSON_HAS_RANGES=0, model TU (Apple clang): −4 %</title></path>
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||||
<text class="nj-charts-val" x="367.6" y="444.5">−4 %</text>
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||||
<path class="nj-charts-gcc" d="M362.6 448 H362.6 Q358.66 448 358.66 451.94 V456.06 Q358.66 460 362.6 460 H362.6 Z"><title>JSON_HAS_RANGES=0, model TU (G​CC 16): −1 %</title></path>
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||||
<text class="nj-charts-val" x="367.6" y="458.5">−1 %</text>
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||||
<line class="nj-charts-zero" x1="362.6" y1="58" x2="362.6" y2="472"/>
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||||
<text class="nj-charts-rowlabel" x="8" y="478"><tspan class="nj-charts-code">JSON_NO_IO</tspan><tspan class="nj-charts-sub"> · model TU*</tspan></text>
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||||
<path class="nj-charts-clang" d="M362.6 486 H358.72 Q354.72 486 354.72 490 V494 Q354.72 498 358.72 498 H362.6 Z"><title>JSON_NO_IO, model TU (Apple clang): −2 %</title></path>
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||||
<text class="nj-charts-val" x="367.6" y="496.5">−2 %</text>
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||||
<path class="nj-charts-gcc" d="M362.6 500 H358.72 Q354.72 500 354.72 504 V508 Q354.72 512 358.72 512 H362.6 Z"><title>JSON_NO_IO, model TU (G​CC 16): −2 %</title></path>
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||||
<text class="nj-charts-val" x="367.6" y="510.5">−2 %</text>
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||||
<text class="nj-charts-rowlabel" x="8" y="530"><tspan class="nj-charts-code">JSON_USE_GLOBAL_UDLS=0</tspan><tspan class="nj-charts-sub"> · model TU*</tspan></text>
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||||
<path class="nj-charts-clang" d="M362.6 538 h0.5 v12 h-0.5 Z"><title>JSON_USE_GLOBAL_UDLS=0, model TU (Apple clang): 0 %</title></path>
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||||
<text class="nj-charts-val" x="367.6" y="548.5">0 %</text>
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||||
<path class="nj-charts-gcc" d="M362.6 552 H362.6 Q358.66 552 358.66 555.94 V560.06 Q358.66 564 362.6 564 H362.6 Z"><title>JSON_USE_GLOBAL_UDLS=0, model TU (G​CC 16): −1 %</title></path>
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||||
<text class="nj-charts-val" x="367.6" y="562.5">−1 %</text>
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||||
<line class="nj-charts-zero" x1="362.6" y1="58" x2="362.6" y2="576"/>
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||||
</svg>
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||||
|
Before Width: | Height: | Size: 7.5 KiB After Width: | Height: | Size: 9.0 KiB |
+124
-24
@@ -11,6 +11,11 @@ that cost, ordered by how much they typically save.
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16.2.0 on macOS (Apple M1 Max). All configurations were run interleaved, in alternating order. They show the order
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of magnitude to expect; measure your own code before and after a change.
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||||
The two sections on `std::format` and ranges apply to C++20 only and were measured with `-std=c++20`, so their
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baselines are slower than the others. Their differences are mostly within the run-to-run noise of a few percent:
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two complete runs of the same configuration differed by up to 10% in the median. The tables show the later run (one
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outlier cell was repeated).
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The translation units are listed below. They replace the ones used for earlier versions of this page, whose
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sources were not published, so the numbers differ from earlier versions.
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@@ -63,24 +68,24 @@ that cost, ordered by how much they typically save.
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```
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<div class="nj-fig-wrap" markdown>
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||||
<svg class="nj-fig nj-charts nj-charts--compile" viewBox="0 0 470 516" role="img" aria-labelledby="nj-charts-ct nj-charts-ct-d" xmlns="http://www.w3.org/2000/svg">
|
||||
<svg class="nj-fig nj-charts nj-charts--compile" viewBox="0 0 470 620" role="img" aria-labelledby="nj-charts-ct nj-charts-ct-d" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="nj-charts-ct">Change in compile time against the default configuration at -O0, Apple clang and G​CC 16</title>
|
||||
<desc id="nj-charts-ct-d">Grouped horizontal bars, negative is faster. json_fwd.hpp instead of json.hpp on a header-only translation unit: Apple clang -62 %, G​CC 16 -82 %; extern template on a parsing translation unit: Apple clang -7 %, G​CC 16 -21 %; JSON_NO_AUTOMATIC_UDLS on a model translation unit: Apple clang -11 %, G​CC 16 -11 %; JSON_NO_AUTOMATIC_UDLS on a parsing translation unit: Apple clang -1 %, G​CC 16 -2 %; JSON_HAS_FILESYSTEM=0 + JSON_NO_IO on a model translation unit: Apple clang -8 %, G​CC 16 -11 %; JSON_HAS_FILESYSTEM=0 on a model translation unit: Apple clang -5 %, G​CC 16 -9 %; JSON_NO_IO on a model translation unit: Apple clang -2 %, G​CC 16 -2 %; JSON_USE_GLOBAL_UDLS=0 on a model translation unit: Apple clang 0 %, G​CC 16 -1 %.</desc>
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||||
<desc id="nj-charts-ct-d">Grouped horizontal bars, negative is faster. json_fwd.hpp instead of json.hpp on a header-only translation unit: Apple clang -62 %, G​CC 16 -82 %; extern template on a parsing translation unit: Apple clang -7 %, G​CC 16 -21 %; JSON_NO_AUTOMATIC_UDLS on a model translation unit: Apple clang -11 %, G​CC 16 -11 %; JSON_NO_AUTOMATIC_UDLS on a parsing translation unit: Apple clang -1 %, G​CC 16 -2 %; JSON_HAS_FILESYSTEM=0 + JSON_NO_IO on a model translation unit: Apple clang -8 %, G​CC 16 -11 %; JSON_HAS_FILESYSTEM=0 on a model translation unit: Apple clang -5 %, G​CC 16 -9 %; JSON_HAS_STD_FORMAT=0 on a model translation unit: Apple clang -1 %, G​CC 16 -4 %; JSON_HAS_RANGES=0 on a model translation unit: Apple clang -4 %, G​CC 16 -1 %; JSON_NO_IO on a model translation unit: Apple clang -2 %, G​CC 16 -2 %; JSON_USE_GLOBAL_UDLS=0 on a model translation unit: Apple clang 0 %, G​CC 16 -1 %.</desc>
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||||
<rect class="nj-charts-clang" x="8" y="10" width="12" height="12" rx="3"/>
|
||||
<text class="nj-charts-leg" x="26" y="20.5">Apple clang</text>
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||||
<rect class="nj-charts-gcc" x="126" y="10" width="12" height="12" rx="3"/>
|
||||
<text class="nj-charts-leg" x="144" y="20.5">G​CC 16</text>
|
||||
<line class="nj-charts-grid" x1="47.4" y1="58" x2="47.4" y2="472"/>
|
||||
<text class="nj-charts-tick" x="47.4" y="488">−80 %</text>
|
||||
<line class="nj-charts-grid" x1="126.2" y1="58" x2="126.2" y2="472"/>
|
||||
<text class="nj-charts-tick" x="126.2" y="488">−60 %</text>
|
||||
<line class="nj-charts-grid" x1="205" y1="58" x2="205" y2="472"/>
|
||||
<text class="nj-charts-tick" x="205" y="488">−40 %</text>
|
||||
<line class="nj-charts-grid" x1="283.8" y1="58" x2="283.8" y2="472"/>
|
||||
<text class="nj-charts-tick" x="283.8" y="488">−20 %</text>
|
||||
<text class="nj-charts-tick" x="362.6" y="488">0 %</text>
|
||||
<text class="nj-charts-tick nj-charts-tick--l" x="8" y="506">← faster</text>
|
||||
<text class="nj-charts-tick nj-charts-tick--r" x="462" y="506">slower →</text>
|
||||
<line class="nj-charts-grid" x1="47.4" y1="58" x2="47.4" y2="576"/>
|
||||
<text class="nj-charts-tick" x="47.4" y="592">−80 %</text>
|
||||
<line class="nj-charts-grid" x1="126.2" y1="58" x2="126.2" y2="576"/>
|
||||
<text class="nj-charts-tick" x="126.2" y="592">−60 %</text>
|
||||
<line class="nj-charts-grid" x1="205" y1="58" x2="205" y2="576"/>
|
||||
<text class="nj-charts-tick" x="205" y="592">−40 %</text>
|
||||
<line class="nj-charts-grid" x1="283.8" y1="58" x2="283.8" y2="576"/>
|
||||
<text class="nj-charts-tick" x="283.8" y="592">−20 %</text>
|
||||
<text class="nj-charts-tick" x="362.6" y="592">0 %</text>
|
||||
<text class="nj-charts-tick nj-charts-tick--l" x="8" y="610">← faster</text>
|
||||
<text class="nj-charts-tick nj-charts-tick--r" x="462" y="610">slower →</text>
|
||||
<text class="nj-charts-rowlabel" x="8" y="62"><tspan class="nj-charts-code">json_fwd.hpp instead of json.hpp</tspan><tspan class="nj-charts-sub"> · header-only TU</tspan></text>
|
||||
<path class="nj-charts-clang" d="M362.6 70 H122.32 Q118.32 70 118.32 74 V78 Q118.32 82 122.32 82 H362.6 Z"><title>json_fwd.hpp instead of json.hpp, header-only TU (Apple clang): −62 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="80.5">−62 %</text>
|
||||
@@ -111,22 +116,33 @@ that cost, ordered by how much they typically save.
|
||||
<text class="nj-charts-val" x="367.6" y="340.5">−5 %</text>
|
||||
<path class="nj-charts-gcc" d="M362.6 344 H331.14 Q327.14 344 327.14 348 V352 Q327.14 356 331.14 356 H362.6 Z"><title>JSON_HAS_FILESYSTEM=0, model TU (G​CC 16): −9 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="354.5">−9 %</text>
|
||||
<text class="nj-charts-rowlabel" x="8" y="374"><tspan class="nj-charts-code">JSON_NO_IO</tspan><tspan class="nj-charts-sub"> · model TU*</tspan></text>
|
||||
<path class="nj-charts-clang" d="M362.6 382 H358.72 Q354.72 382 354.72 386 V390 Q354.72 394 358.72 394 H362.6 Z"><title>JSON_NO_IO, model TU (Apple clang): −2 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="392.5">−2 %</text>
|
||||
<path class="nj-charts-gcc" d="M362.6 396 H358.72 Q354.72 396 354.72 400 V404 Q354.72 408 358.72 408 H362.6 Z"><title>JSON_NO_IO, model TU (G​CC 16): −2 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="406.5">−2 %</text>
|
||||
<text class="nj-charts-rowlabel" x="8" y="426"><tspan class="nj-charts-code">JSON_USE_GLOBAL_UDLS=0</tspan><tspan class="nj-charts-sub"> · model TU*</tspan></text>
|
||||
<path class="nj-charts-clang" d="M362.6 434 h0.5 v12 h-0.5 Z"><title>JSON_USE_GLOBAL_UDLS=0, model TU (Apple clang): 0 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="444.5">0 %</text>
|
||||
<path class="nj-charts-gcc" d="M362.6 448 H362.6 Q358.66 448 358.66 451.94 V456.06 Q358.66 460 362.6 460 H362.6 Z"><title>JSON_USE_GLOBAL_UDLS=0, model TU (G​CC 16): −1 %</title></path>
|
||||
<text class="nj-charts-rowlabel" x="8" y="374"><tspan class="nj-charts-code">JSON_HAS_STD_FORMAT=0</tspan><tspan class="nj-charts-sub"> · model TU</tspan></text>
|
||||
<path class="nj-charts-clang" d="M362.6 382 H362.6 Q358.66 382 358.66 385.94 V390.06 Q358.66 394 362.6 394 H362.6 Z"><title>JSON_HAS_STD_FORMAT=0, model TU (Apple clang): −1 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="392.5">−1 %</text>
|
||||
<path class="nj-charts-gcc" d="M362.6 396 H350.84 Q346.84 396 346.84 400 V404 Q346.84 408 350.84 408 H362.6 Z"><title>JSON_HAS_STD_FORMAT=0, model TU (G​CC 16): −4 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="406.5">−4 %</text>
|
||||
<text class="nj-charts-rowlabel" x="8" y="426"><tspan class="nj-charts-code">JSON_HAS_RANGES=0</tspan><tspan class="nj-charts-sub"> · model TU</tspan></text>
|
||||
<path class="nj-charts-clang" d="M362.6 434 H350.84 Q346.84 434 346.84 438 V442 Q346.84 446 350.84 446 H362.6 Z"><title>JSON_HAS_RANGES=0, model TU (Apple clang): −4 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="444.5">−4 %</text>
|
||||
<path class="nj-charts-gcc" d="M362.6 448 H362.6 Q358.66 448 358.66 451.94 V456.06 Q358.66 460 362.6 460 H362.6 Z"><title>JSON_HAS_RANGES=0, model TU (G​CC 16): −1 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="458.5">−1 %</text>
|
||||
<line class="nj-charts-zero" x1="362.6" y1="58" x2="362.6" y2="472"/>
|
||||
<text class="nj-charts-rowlabel" x="8" y="478"><tspan class="nj-charts-code">JSON_NO_IO</tspan><tspan class="nj-charts-sub"> · model TU*</tspan></text>
|
||||
<path class="nj-charts-clang" d="M362.6 486 H358.72 Q354.72 486 354.72 490 V494 Q354.72 498 358.72 498 H362.6 Z"><title>JSON_NO_IO, model TU (Apple clang): −2 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="496.5">−2 %</text>
|
||||
<path class="nj-charts-gcc" d="M362.6 500 H358.72 Q354.72 500 354.72 504 V508 Q354.72 512 358.72 512 H362.6 Z"><title>JSON_NO_IO, model TU (G​CC 16): −2 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="510.5">−2 %</text>
|
||||
<text class="nj-charts-rowlabel" x="8" y="530"><tspan class="nj-charts-code">JSON_USE_GLOBAL_UDLS=0</tspan><tspan class="nj-charts-sub"> · model TU*</tspan></text>
|
||||
<path class="nj-charts-clang" d="M362.6 538 h0.5 v12 h-0.5 Z"><title>JSON_USE_GLOBAL_UDLS=0, model TU (Apple clang): 0 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="548.5">0 %</text>
|
||||
<path class="nj-charts-gcc" d="M362.6 552 H362.6 Q358.66 552 358.66 555.94 V560.06 Q358.66 564 362.6 564 H362.6 Z"><title>JSON_USE_GLOBAL_UDLS=0, model TU (G​CC 16): −1 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="562.5">−1 %</text>
|
||||
<line class="nj-charts-zero" x1="362.6" y1="58" x2="362.6" y2="576"/>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
<p class="nj-charts-caption" markdown>Change in compile time at `-O0` against the default configuration of the same
|
||||
translation unit, taken from the tables below (medians of nine runs, see above). * computed from the milliseconds in
|
||||
translation unit, taken from the tables below (medians of nine runs, see above; the `std::format` and ranges rows are
|
||||
measured with `-std=c++20`). * computed from the milliseconds in
|
||||
[Options without effect on compile times](#options-without-effect-on-compile-times).</p>
|
||||
|
||||
## Include `json_fwd.hpp` in headers
|
||||
@@ -274,6 +290,88 @@ from and to `std::string`; whether that fallback exists depends on the platform'
|
||||
(`std::wstring` on Windows), so check code that converts paths. `JSON_NO_IO` additionally removes all stream
|
||||
functions (see [Options without effect](#options-without-effect-on-compile-times)).
|
||||
|
||||
## Switch off `std::format` support
|
||||
|
||||
With C++20 and a standard library that provides `<format>`, the library includes it to provide a
|
||||
[`std::formatter<basic_json>`](../api/basic_json/std_formatter.md) specialization (see
|
||||
[`JSON_HAS_STD_FORMAT`](../api/macros/json_has_std_format.md)). If you do not format JSON values with `std::format`,
|
||||
switch the detection off for the whole project:
|
||||
|
||||
```cmake
|
||||
target_compile_definitions(my_target PRIVATE JSON_HAS_STD_FORMAT=0)
|
||||
```
|
||||
|
||||
| Compiler | Translation unit | Default (`-O0` / `-O2`) | `std::format` off (`-O0` / `-O2`) | Change |
|
||||
|-------------|------------------|------------------------:|----------------------------------:|-----------:|
|
||||
| Apple clang | model | 1283 ms / 1544 ms | 1269 ms / 1539 ms | −1% / 0% |
|
||||
| GCC 16 | model | 2026 ms / 2482 ms | 1944 ms / 2416 ms | −4% / −3% |
|
||||
| Apple clang | parsing | 1350 ms / 2278 ms | 1347 ms / 2300 ms | 0% / +1% |
|
||||
| GCC 16 | parsing | 2715 ms / 4351 ms | 2644 ms / 4287 ms | −3% / −1% |
|
||||
|
||||
<p class="nj-table-caption" markdown>Compile time with `JSON_HAS_STD_FORMAT=0` against the default configuration, both with `-std=c++20`, at `-O0` and `-O2`.</p>
|
||||
|
||||
The saving is small, and only GCC shows it: libstdc++'s `<format>` is an additional header that nothing else in the
|
||||
translation unit includes, so the macro removes it (and the headers it pulls in). With libc++ (Apple clang), other
|
||||
standard headers include `<format>` anyway, so the macro only removes the library's own formatter, and the difference is
|
||||
within the noise.
|
||||
|
||||
The trade-off: the `std::formatter<basic_json>` specialization is removed, so `#!cpp std::format("{}", j)`,
|
||||
and `#!cpp std::format_to` with a JSON value no longer compile (in a test with both compilers on
|
||||
macOS, the error was that `std::formatter` must be specialized for the type). Use [`dump()`](../api/basic_json/dump.md),
|
||||
or the stream operators, instead.
|
||||
|
||||
## Switch off ranges support
|
||||
|
||||
With C++20 and a standard library that provides `<ranges>`, the library includes it to declare the
|
||||
[`items()`](../api/basic_json/items.md) proxy a *borrowed range*, and uses the `std::ranges` concepts and algorithms for
|
||||
construction from range views and for iterator input (see [`JSON_HAS_RANGES`](../api/macros/json_has_ranges.md)). If you
|
||||
do not need these, switch the detection off for the whole project:
|
||||
|
||||
```cmake
|
||||
target_compile_definitions(my_target PRIVATE JSON_HAS_RANGES=0)
|
||||
```
|
||||
|
||||
| Compiler | Translation unit | Default (`-O0` / `-O2`) | Ranges off (`-O0` / `-O2`) | Change |
|
||||
|-------------|------------------|------------------------:|---------------------------:|-----------:|
|
||||
| Apple clang | model | 1283 ms / 1544 ms | 1237 ms / 1495 ms | −4% / −3% |
|
||||
| GCC 16 | model | 2026 ms / 2482 ms | 2004 ms / 2464 ms | −1% / −1% |
|
||||
| Apple clang | parsing | 1350 ms / 2278 ms | 1311 ms / 2284 ms | −3% / 0% |
|
||||
| GCC 16 | parsing | 2715 ms / 4351 ms | 2705 ms / 4427 ms | 0% / +2% |
|
||||
|
||||
<p class="nj-table-caption" markdown>Compile time with `JSON_HAS_RANGES=0` against the default configuration, both with `-std=c++20`, at `-O0` and `-O2`.</p>
|
||||
|
||||
The saving is small as well, and with GCC it is not distinguishable from the noise: its `<format>` includes `<span>`,
|
||||
so with `std::format` support left on, part of the ranges machinery is still parsed.
|
||||
Both macros together save more than either alone:
|
||||
|
||||
```cmake
|
||||
target_compile_definitions(my_target PRIVATE JSON_HAS_STD_FORMAT=0 JSON_HAS_RANGES=0)
|
||||
```
|
||||
|
||||
| Compiler | Translation unit | Default (`-O0` / `-O2`) | Both off (`-O0` / `-O2`) | Change |
|
||||
|-------------|------------------|------------------------:|-------------------------:|-----------:|
|
||||
| Apple clang | model | 1283 ms / 1544 ms | 1223 ms / 1504 ms | −5% / −3% |
|
||||
| GCC 16 | model | 2026 ms / 2482 ms | 1914 ms / 2378 ms | −6% / −4% |
|
||||
| Apple clang | parsing | 1350 ms / 2278 ms | 1311 ms / 2271 ms | −3% / 0% |
|
||||
| GCC 16 | parsing | 2715 ms / 4351 ms | 2635 ms / 4260 ms | −3% / −2% |
|
||||
|
||||
<p class="nj-table-caption" markdown>Compile time with `JSON_HAS_STD_FORMAT=0` and `JSON_HAS_RANGES=0` against the default configuration, both with `-std=c++20`, at `-O0` and `-O2`.</p>
|
||||
|
||||
The trade-offs of `JSON_HAS_RANGES=0` are larger than those of the other options on this page:
|
||||
|
||||
- `items()` is no longer a borrowed range. On a temporary, `#!cpp std::ranges::begin(make_json().items())` no longer
|
||||
compiles, and `std::ranges` algorithms such as `#!cpp std::ranges::find_if(make_json().items(), pred)` return
|
||||
`std::ranges::dangling` instead of an iterator, so code that uses the result no longer compiles. Calling them on
|
||||
`#!cpp j.items()` for a named `j` is not affected.
|
||||
- [`JSON_HAS_RANGE_VIEW_CONVERSION`](../api/macros/json_has_range_view_conversion.md) becomes `0`, so a JSON array can no
|
||||
longer be constructed from a `std::ranges::view` such as the result of `std::views::filter`. The assignment
|
||||
`#!cpp json j = v | std::views::filter(pred);` no longer compiles.
|
||||
- Parsing from an iterator pair loses the C++20 fast path for contiguous iterators that are not pointers (for
|
||||
example those of `std::vector`) and falls back to byte-at-a-time scanning. The result is the same; parsing may be
|
||||
slower (not measured here).
|
||||
|
||||
The first two were checked with both compilers on macOS; the third is read from the library's source.
|
||||
|
||||
## Use C++20 modules
|
||||
|
||||
With a toolchain that supports named modules, `import nlohmann.json;` compiles the library once into a module and
|
||||
@@ -463,6 +561,8 @@ and −2% / 0% (GCC). To get a saving, combine it with
|
||||
|
||||
- [`JSON_HAS_FILESYSTEM`](../api/macros/json_has_filesystem.md) - control the conversions from and to
|
||||
`std::filesystem::path`
|
||||
- [`JSON_HAS_STD_FORMAT`](../api/macros/json_has_std_format.md) - control the `std::formatter` specialization
|
||||
- [`JSON_HAS_RANGES`](../api/macros/json_has_ranges.md) - control the use of `std::ranges`
|
||||
- [`JSON_NO_AUTOMATIC_UDLS`](../api/macros/json_no_automatic_udls.md) - do not include the user-defined string
|
||||
literals automatically
|
||||
- [Modules](../features/modules.md) - C++20 module support
|
||||
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/integration/compile_times/" rel="canonical"><link href=../pkg-config/ rel=prev><link href=../../api/basic_json/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Compile times - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet href=../../css/fig-exceptions.css><link rel=stylesheet href=../../css/fig-pointer.css><link rel=stylesheet href=../../css/fig-charts.css><link rel=stylesheet href=../../css/fig-integration.css><link rel=stylesheet href=../../css/fig-macros.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#compile-times class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Compile times </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.Line truncated
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/integration/compile_times/" rel="canonical"><link href=../pkg-config/ rel=prev><link href=../../api/basic_json/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Compile times - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet href=../../css/fig-exceptions.css><link rel=stylesheet href=../../css/fig-pointer.css><link rel=stylesheet href=../../css/fig-charts.css><link rel=stylesheet href=../../css/fig-integration.css><link rel=stylesheet href=../../css/fig-macros.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#compile-times class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Compile times </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.Line truncated
|
||||
<span class=p>{</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>name</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=kt>int</span><span class=w> </span><span class=n>age</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>0</span><span class=p>;</span>
|
||||
@@ -32,7 +32,7 @@
|
||||
<span class=k>struct</span><span class=w> </span><span class=nc>person</span><span class=p>;</span>
|
||||
<span class=kt>void</span><span class=w> </span><span class=nf>to_json</span><span class=p>(</span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=o>&</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>person</span><span class=o>&</span><span class=w> </span><span class=n>p</span><span class=p>);</span>
|
||||
<span class=kt>void</span><span class=w> </span><span class=nf>from_json</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=o>&</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=n>person</span><span class=o>&</span><span class=w> </span><span class=n>p</span><span class=p>);</span>
|
||||
</code></pre></div> </details> <div class=nj-fig-wrap> <p><svg class="nj-fig nj-charts nj-charts--compile" viewbox="0 0 470 516" role=img aria-labelledby="nj-charts-ct nj-charts-ct-d" xmlns=http://www.w3.org/2000/svg> <title id=nj-charts-ct>Change in compile time against the default configuration at -O0, Apple clang and G​CC 16</title> <desc id=nj-charts-ct-d>Grouped horizontal bars, negative is faster. json_fwd.hpp instead of json.hpp on a header-only translation unit: Apple clang -62 %, G​CC 16 -82 %; extern template on a parsing translation unit: Apple clang -7 %, G​CC 16 -21 %; JSON_NO_AUTOMATIC_UDLS on a model translation unit: Apple clang -11 %, G​CC 16 -11 %; JSON_NO_AUTOMATIC_UDLS on a parsing translation unit: Apple clang -1 %, G​CC 16 -2 %; JSON_HAS_FILESYSTEM=0 + JSON_NO_IO on a model translation unit: Apple clang -8 %, G​CC 16 -11 %; JSON_HAS_FILESYSTEM=0 on a model translation unit: Apple clang -5 %, G​CC 16 -9 %; JSON_NO_IO on a model translation unit: Apple clang -2 %, G​CC 16 -2 %; JSON_USE_GLOBAL_UDLS=0 on a model translation unit: Apple clang 0 %, G​CC 16 -1 %.</desc> <rect class=nj-charts-clang x=8 y=10 width=12 height=12 rx=3 /> <text class=nj-charts-leg x=26 y=20.5>Apple clang</text> <rect class=nj-charts-gcc x=126 y=10 width=12 height=12 rx=3 /> <text class=nj-charts-leg x=144 y=20.5>G​CC 16</text> <line class=nj-charts-grid x1=47.4 y1=58 x2=47.4 y2=472 /> <text class=nj-charts-tick x=47.4 y=488>−80 %</text> <line class=nj-charts-grid x1=126.2 y1=58 x2=126.2 y2=472 /> <text class=nj-charts-tick x=126.2 y=488>−60 %</text> <line class=nj-charts-grid x1=205 y1=58 x2=205 y2=472 /> <text class=nj-charts-tick x=205 y=488>−40 %</text> <line class=nj-charts-grid x1=283.8 y1=58 x2=283.8 y2=472 /> <text class=nj-charts-tick x=283.8 y=488>−20 %</text> <text class=nj-charts-tick x=362.6 y=488>0 %</text> <text class="nj-charts-tick nj-charts-tick--l" x=8 y=506>← faster</text> <text class="nj-charts-tick nj-charts-tick--r" x=462 y=506>slower →</text> <text class=nj-charts-rowlabel x=8 y=62><tspan class=nj-charts-code>json_fwd.hpp instead of json.hpp</tspan><tspan class=nj-charts-sub> · header-only TU</tspan></text> <path class=nj-charts-clang d="M362.6 70 H122.32 Q118.32 70 118.32 74 V78 Q118.32 82 122.32 82 H362.6 Z"><title>json_fwd.hpp instead of json.hpp, header-only TU (Apple clang): −62 %</title></path> <text class=nj-charts-val x=367.6 y=80.5>−62 %</text> <path class=nj-charts-gcc d="M362.6 84 H43.52 Q39.52 84 39.52 88 V92 Q39.52 96 43.52 96 H362.6 Z"><title>json_fwd.hpp instead of json.hpp, header-only TU (G​CC 16): −82 %</title></path> <text class=nj-charts-val x=367.6 y=94.5>−82 %</text> <text class=nj-charts-rowlabel x=8 y=114><tspan class=nj-charts-code>extern template</tspan><tspan class=nj-charts-sub> · parsing TU</tspan></text> <path class=nj-charts-clang d="M362.6 122 H339.02 Q335.02 122 335.02 126 V130 Q335.02 134 339.02 134 H362.6 Z"><title>extern template, parsing TU (Apple clang): −7 %</title></path> <text class=nj-charts-val x=367.6 y=132.5>−7 %</text> <path class=nj-charts-gcc d="M362.6 136 H283.86 Q279.86 136 279.86 140 V144 Q279.86 148 283.86 148 H362.6 Z"><title>extern template, parsing TU (G​CC 16): −21 %</title></path> <text class=nj-charts-val x=367.6 y=146.5>−21 %</text> <text class=nj-charts-rowlabel x=8 y=166><tspan class=nj-charts-code>JSON_NO_AUTOMATIC_UDLS</tspan><tspan class=nj-charts-sub> · model TU</tspan></text> <path class=nj-charts-clang d="M362.6 174 H323.26 Q319.26 174 319.26 178 V182 Q319.26 186 323.26 186 H362.6 Z"><title>JSON_NO_AUTOMATIC_UDLS, model TU (Apple clang): −11 %</title></path> <text class=nj-charts-val x=367.6 y=184.5>−11 %</text> <path class=nj-charts-gcc d="M362.6 188 H323.26 Q319.26 188 319.26 192 V196 Q319.26 200 323.26 200 H362.6 Z"><title>JSON_NO_AUTOMATIC_UDLS, model TU (G​CC 16): −11 %</title></path> <text class=nj-charts-val x=367.6 y=198.5>−11 %</text> <text class=nj-charts-rowlabel x=8 y=218><tspan class=nj-charts-code>JSON_NO_AUTOMATIC_UDLS</tspan><tspan class=nj-charts-sub> · parsing TU</tspan></text> <path class=nj-charts-clang d="M362.6 226 H362.6 Q358.66 226 358.66 229.94 V234.06 Q358.66 238 362.6 238 H362.6 Z"><title>JSON_NO_AUTOMATIC_UDLS, parsing TU (Apple clang): −1 %</title></path> <text class=nj-charts-val x=367.6 y=236.5>−1 %</text> <path class=nj-charts-gcc d="M362.6 240 H358.72 Q354.72 240 354.72 244 V248 Q354.72 252 358.72 252 H362.6 Z"><title>JSON_NO_AUTOMATIC_UDLS, parsing TU (G​CC 16): −2 %</title></path> <text class=nj-charts-val x=367.6 y=250.5>−2 %</text> <text class=nj-charts-rowlabel x=8 y=270><tspan class=nj-charts-code>JSON_HAS_FILESYSTEM=0 + JSON_NO_IO</tspan><tspan class=nj-charts-sub> · model TU</tspan></text> <path class=nj-charts-clang d="M362.6 278 H335.08 Q331.08 2Line truncated
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</code></pre></div> </details> <div class=nj-fig-wrap> <p><svg class="nj-fig nj-charts nj-charts--compile" viewbox="0 0 470 620" role=img aria-labelledby="nj-charts-ct nj-charts-ct-d" xmlns=http://www.w3.org/2000/svg> <title id=nj-charts-ct>Change in compile time against the default configuration at -O0, Apple clang and G​CC 16</title> <desc id=nj-charts-ct-d>Grouped horizontal bars, negative is faster. json_fwd.hpp instead of json.hpp on a header-only translation unit: Apple clang -62 %, G​CC 16 -82 %; extern template on a parsing translation unit: Apple clang -7 %, G​CC 16 -21 %; JSON_NO_AUTOMATIC_UDLS on a model translation unit: Apple clang -11 %, G​CC 16 -11 %; JSON_NO_AUTOMATIC_UDLS on a parsing translation unit: Apple clang -1 %, G​CC 16 -2 %; JSON_HAS_FILESYSTEM=0 + JSON_NO_IO on a model translation unit: Apple clang -8 %, G​CC 16 -11 %; JSON_HAS_FILESYSTEM=0 on a model translation unit: Apple clang -5 %, G​CC 16 -9 %; JSON_HAS_STD_FORMAT=0 on a model translation unit: Apple clang -1 %, G​CC 16 -4 %; JSON_HAS_RANGES=0 on a model translation unit: Apple clang -4 %, G​CC 16 -1 %; JSON_NO_IO on a model translation unit: Apple clang -2 %, G​CC 16 -2 %; JSON_USE_GLOBAL_UDLS=0 on a model translation unit: Apple clang 0 %, G​CC 16 -1 %.</desc> <rect class=nj-charts-clang x=8 y=10 width=12 height=12 rx=3 /> <text class=nj-charts-leg x=26 y=20.5>Apple clang</text> <rect class=nj-charts-gcc x=126 y=10 width=12 height=12 rx=3 /> <text class=nj-charts-leg x=144 y=20.5>G​CC 16</text> <line class=nj-charts-grid x1=47.4 y1=58 x2=47.4 y2=576 /> <text class=nj-charts-tick x=47.4 y=592>−80 %</text> <line class=nj-charts-grid x1=126.2 y1=58 x2=126.2 y2=576 /> <text class=nj-charts-tick x=126.2 y=592>−60 %</text> <line class=nj-charts-grid x1=205 y1=58 x2=205 y2=576 /> <text class=nj-charts-tick x=205 y=592>−40 %</text> <line class=nj-charts-grid x1=283.8 y1=58 x2=283.8 y2=576 /> <text class=nj-charts-tick x=283.8 y=592>−20 %</text> <text class=nj-charts-tick x=362.6 y=592>0 %</text> <text class="nj-charts-tick nj-charts-tick--l" x=8 y=610>← faster</text> <text class="nj-charts-tick nj-charts-tick--r" x=462 y=610>slower →</text> <text class=nj-charts-rowlabel x=8 y=62><tspan class=nj-charts-code>json_fwd.hpp instead of json.hpp</tspan><tspan class=nj-charts-sub> · header-only TU</tspan></text> <path class=nj-charts-clang d="M362.6 70 H122.32 Q118.32 70 118.32 74 V78 Q118.32 82 122.32 82 H362.6 Z"><title>json_fwd.hpp instead of json.hpp, header-only TU (Apple clang): −62 %</title></path> <text class=nj-charts-val x=367.6 y=80.5>−62 %</text> <path class=nj-charts-gcc d="M362.6 84 H43.52 Q39.52 84 39.52 88 V92 Q39.52 96 43.52 96 H362.6 Z"><title>json_fwd.hpp instead of json.hpp, header-only TU (G​CC 16): −82 %</title></path> <text class=nj-charts-val x=367.6 y=94.5>−82 %</text> <text class=nj-charts-rowlabel x=8 y=114><tspan class=nj-charts-code>extern template</tspan><tspan class=nj-charts-sub> · parsing TU</tspan></text> <path class=nj-charts-clang d="M362.6 122 H339.02 Q335.02 122 335.02 126 V130 Q335.02 134 339.02 134 H362.6 Z"><title>extern template, parsing TU (Apple clang): −7 %</title></path> <text class=nj-charts-val x=367.6 y=132.5>−7 %</text> <path class=nj-charts-gcc d="M362.6 136 H283.86 Q279.86 136 279.86 140 V144 Q279.86 148 283.86 148 H362.6 Z"><title>extern template, parsing TU (G​CC 16): −21 %</title></path> <text class=nj-charts-val x=367.6 y=146.5>−21 %</text> <text class=nj-charts-rowlabel x=8 y=166><tspan class=nj-charts-code>JSON_NO_AUTOMATIC_UDLS</tspan><tspan class=nj-charts-sub> · model TU</tspan></text> <path class=nj-charts-clang d="M362.6 174 H323.26 Q319.26 174 319.26 178 V182 Q319.26 186 323.26 186 H362.6 Z"><title>JSON_NO_AUTOMATIC_UDLS, model TU (Apple clang): −11 %</title></path> <text class=nj-charts-val x=367.6 y=184.5>−11 %</text> <path class=nj-charts-gcc d="M362.6 188 H323.26 Q319.26 188 319.26 192 V196 Q319.26 200 323.26 200 H362.6 Z"><title>JSON_NO_AUTOMATIC_UDLS, model TU (G​CC 16): −11 %</title></path> <text class=nj-charts-val x=367.6 y=198.5>−11 %</text> <text class=nj-charts-rowlabel x=8 y=218><tspan class=nj-charts-code>JSON_NO_AUTOMATIC_UDLS</tspan><tspan class=nj-charts-sub> · parsing TU</tspan></text> <path class=nj-charts-clang d="M362.6 226 H362.6 Q358.66 226 358.66 229.94 V234.06 Q358.66 238 362.6 238 H362.6 Z"><title>JSON_NO_AUTOMATIC_UDLS, parsing TU (Apple clang): −1 %</title></path> <text class=nj-charts-val x=367.6 y=236.5>−1 %</text> <path class=nj-charts-gcc d="M362.6 240 H358.72 Q354.72 240 354.72 244 V248 Q354.72 252 358.72 252 H362.6 Z"><title>JSON_NO_AUTOMATIC_UDLS, parsing TU (G​CC 16): −2 %</title></path> <text class=nj-charts-val x=367.6 y=250.5>−2 %</text> <text class=nj-charts-rowlabel x=8 y=270><tspan class=nLine truncated
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<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json_fwd.hpp></span>
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<span class=k>struct</span><span class=w> </span><span class=nc>person</span><span class=p>;</span>
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@@ -53,7 +53,10 @@
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<span class=k>template</span><span class=w> </span><span class=k>class</span><span class=w> </span><span class=nc>nlohmann</span><span class=o>::</span><span class=n>basic_json</span><span class=o><></span><span class=p>;</span>
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</code></pre></div> <p>Include <code>json_instance.hpp</code> instead of <code><nlohmann/json.hpp></code> and compile and link <code>json_instance.cpp</code> once.</p> <table> <thead> <tr> <th>Compiler</th> <th>Translation unit</th> <th style="text-align: right;">Default (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;"><code>extern template</code> (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;">Change</th> </tr> </thead> <tbody> <tr> <td>Apple clang</td> <td>parsing</td> <td style="text-align: right;">1157 ms / 2016 ms</td> <td style="text-align: right;">1071 ms / 1844 ms</td> <td style="text-align: right;">−7% / −9%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>parsing</td> <td style="text-align: right;">2529 ms / 4087 ms</td> <td style="text-align: right;">1994 ms / 3190 ms</td> <td style="text-align: right;">−21% / −22%</td> </tr> <tr> <td>Apple clang</td> <td><code>json_instance.cpp</code></td> <td style="text-align: right;">—</td> <td style="text-align: right;">2573 ms / 6221 ms</td> <td style="text-align: right;">—</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td><code>json_instance.cpp</code></td> <td style="text-align: right;">—</td> <td style="text-align: right;">6489 ms / 13759 ms</td> <td style="text-align: right;">—</td> </tr> </tbody> </table> <p class=nj-table-caption>Compile time of the parsing translation unit without and with the <code>extern template</code> declaration, at <code>-O0</code> and <code>-O2</code>. The <code>json_instance.cpp</code> rows are the one-off cost of the translation unit that holds the instantiation.</p> <p>Notes:</p> <ul> <li>The saving grows with the number of translation units that use <code>json</code>, while the instantiation translation unit is compiled only once (and is rarely recompiled, as it does not depend on your code).</li> <li>Member function templates (such as <code>get<T>()</code>, <code>parse(InputType&&)</code>, or <code>value(key, default)</code>) are not covered by the explicit instantiation and are still instantiated where they are used.</li> <li>The declaration covers exactly <code>nlohmann::json</code>. Add the same lines for <code>nlohmann::ordered_json</code> (<code>nlohmann::basic_json<nlohmann::ordered_map></code>) or your own <code>basic_json</code> specializations if you use them.</li> </ul> <h2 id=switch-off-filesystem-support>Switch off filesystem support<a class=headerlink href=#switch-off-filesystem-support title="Permanent link">¶</a></h2> <p>With C++17, the library includes <code><filesystem></code> to provide conversions from and to <code>std::filesystem::path</code> (see <a href=../../api/macros/json_has_filesystem/ ><code>JSON_HAS_FILESYSTEM</code></a>). Parsing <code><filesystem></code> is a fixed cost in every translation unit, and it also pulls in <code><iomanip></code>, <code><istream></code>, and <code><ostream></code> (and, with <abbr title="GNU Compiler Collection">GCC</abbr>'s libstdc++, <code><sstream></code>). If you do not convert paths, switch the detection off for the whole project:</p> <div class=highlight><pre><span></span><code><span class=nb>target_compile_definitions</span><span class=p>(</span><span class=s>my_target</span><span class=w> </span><span class=s>PRIVATE</span><span class=w> </span><span class=s>JSON_HAS_FILESYSTEM=0</span><span class=w> </span><span class=s>JSON_HAS_EXPERIMENTAL_FILESYSTEM=0</span><span class=p>)</span>
|
||||
</code></pre></div> <table> <thead> <tr> <th>Compiler</th> <th>Translation unit</th> <th style="text-align: right;">Default (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;">Filesystem off (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;">Change</th> </tr> </thead> <tbody> <tr> <td>Apple clang</td> <td>model</td> <td style="text-align: right;">1051 ms / 1300 ms</td> <td style="text-align: right;">998 ms / 1257 ms</td> <td style="text-align: right;">−5% / −3%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>model</td> <td style="text-align: right;">1801 ms / 2106 ms</td> <td style="text-align: right;">1636 ms / 2008 ms</td> <td style="text-align: right;">−9% / −5%</td> </tr> <tr> <td>Apple clang</td> <td>parsing</td> <td style="text-align: right;">1157 ms / 2016 ms</td> <td style="text-align: right;">1107 ms / 1971 ms</td> <td style="text-align: right;">−4% / −2%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>parsing</td> <td style="text-align: right;">2529 ms / 4087 ms</td> <td style="text-align: right;">2405 ms / 4029 ms</td> <td style="text-align: right;">−5% / −1%</td> </tr> </tbody> </table> <p class=nj-table-caption>Compile time with <code>JSON_HAS_FILESYSTEM=0</code> and <code>JSON_HAS_EXPERIMENTAL_FILESYSTEM=0</code> against the default configuration, at <code>-O0</code> and <code>-O2</code>.</p> <p>Only with filesystem support switched off does <a href=../../api/macros/json_no_io/ ><code>JSON_NO_IO</code></a> remove the stream headers from the translation unit, so the two macros save most together:</p> <table> <thead> <tr> <th>Compiler</th> <th>Translation unit</th> <th style="text-align: right;">Default (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;">Filesystem off and <code>JSON_NO_IO</code> (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;">Change</th> </tr> </thead> <tbody> <tr> <td>Apple clang</td> <td>model</td> <td style="text-align: right;">1051 ms / 1300 ms</td> <td style="text-align: right;">969 ms / 1219 ms</td> <td style="text-align: right;">−8% / −6%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>model</td> <td style="text-align: right;">1801 ms / 2106 ms</td> <td style="text-align: right;">1605 ms / 1982 ms</td> <td style="text-align: right;">−11% / −6%</td> </tr> <tr> <td>Apple clang</td> <td>parsing</td> <td style="text-align: right;">1157 ms / 2016 ms</td> <td style="text-align: right;">1083 ms / 1947 ms</td> <td style="text-align: right;">−6% / −3%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>parsing</td> <td style="text-align: right;">2529 ms / 4087 ms</td> <td style="text-align: right;">2384 ms / 3972 ms</td> <td style="text-align: right;">−6% / −3%</td> </tr> </tbody> </table> <p class=nj-table-caption>Compile time with filesystem support switched off and <code>JSON_NO_IO</code> defined, against the default configuration, at <code>-O0</code> and <code>-O2</code>.</p> <p>The trade-off: the library's own <code>to_json</code>/<code>from_json</code> overloads for <code>std::filesystem::path</code> (which store the path as <abbr title="Unicode Transformation Format">UTF</abbr>-8) are removed. In a test with both compilers on macOS, paths still converted through their implicit conversion from and to <code>std::string</code>; whether that fallback exists depends on the platform's native path string type (<code>std::wstring</code> on Windows), so check code that converts paths. <code>JSON_NO_IO</code> additionally removes all stream functions (see <a href=#options-without-effect-on-compile-times>Options without effect</a>).</p> <h2 id=use-c20-modules>Use C++20 modules<a class=headerlink href=#use-c20-modules title="Permanent link">¶</a></h2> <p>With a toolchain that supports named modules, <code>import nlohmann.json;</code> compiles the library once into a module and avoids parsing the header in every translation unit. See <a href=../../features/modules/ >Modules</a> for requirements and known issues. Module support is experimental and currently depends heavily on the compiler version.</p> <div class=nj-fig-wrap> <p><svg class="nj-fig nj-charts nj-charts--compile" viewbox="0 0 440 282" role=img aria-labelledby="nj-charts-mod nj-charts-mod-d" xmlns=http://www.w3.org/2000/svg> <title id=nj-charts-mod>Compile time per translation unit with import nlohmann.json at -O0, and the one-off cost of the module</title> <desc id=nj-charts-mod-d>Horizontal bars, negative is faster, medians of nine runs, -std=c++20 for both. model TU, Apple clang: -48 % (1311 ms to 680 ms); model TU, G​CC 16: -47 % (2156 ms to 1153 ms); parsing TU, Apple clang: -42 % (1387 ms to 800 ms); parsing TU, G​CC 16: -36 % (2838 ms to 1810 ms); one-off build, Apple clang: 1630 ms; one-off build, G​CC 16: 2538 ms.</desc> <rect class=nj-charts-cLine truncated
|
||||
</code></pre></div> <table> <thead> <tr> <th>Compiler</th> <th>Translation unit</th> <th style="text-align: right;">Default (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;">Filesystem off (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;">Change</th> </tr> </thead> <tbody> <tr> <td>Apple clang</td> <td>model</td> <td style="text-align: right;">1051 ms / 1300 ms</td> <td style="text-align: right;">998 ms / 1257 ms</td> <td style="text-align: right;">−5% / −3%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>model</td> <td style="text-align: right;">1801 ms / 2106 ms</td> <td style="text-align: right;">1636 ms / 2008 ms</td> <td style="text-align: right;">−9% / −5%</td> </tr> <tr> <td>Apple clang</td> <td>parsing</td> <td style="text-align: right;">1157 ms / 2016 ms</td> <td style="text-align: right;">1107 ms / 1971 ms</td> <td style="text-align: right;">−4% / −2%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>parsing</td> <td style="text-align: right;">2529 ms / 4087 ms</td> <td style="text-align: right;">2405 ms / 4029 ms</td> <td style="text-align: right;">−5% / −1%</td> </tr> </tbody> </table> <p class=nj-table-caption>Compile time with <code>JSON_HAS_FILESYSTEM=0</code> and <code>JSON_HAS_EXPERIMENTAL_FILESYSTEM=0</code> against the default configuration, at <code>-O0</code> and <code>-O2</code>.</p> <p>Only with filesystem support switched off does <a href=../../api/macros/json_no_io/ ><code>JSON_NO_IO</code></a> remove the stream headers from the translation unit, so the two macros save most together:</p> <table> <thead> <tr> <th>Compiler</th> <th>Translation unit</th> <th style="text-align: right;">Default (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;">Filesystem off and <code>JSON_NO_IO</code> (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;">Change</th> </tr> </thead> <tbody> <tr> <td>Apple clang</td> <td>model</td> <td style="text-align: right;">1051 ms / 1300 ms</td> <td style="text-align: right;">969 ms / 1219 ms</td> <td style="text-align: right;">−8% / −6%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>model</td> <td style="text-align: right;">1801 ms / 2106 ms</td> <td style="text-align: right;">1605 ms / 1982 ms</td> <td style="text-align: right;">−11% / −6%</td> </tr> <tr> <td>Apple clang</td> <td>parsing</td> <td style="text-align: right;">1157 ms / 2016 ms</td> <td style="text-align: right;">1083 ms / 1947 ms</td> <td style="text-align: right;">−6% / −3%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>parsing</td> <td style="text-align: right;">2529 ms / 4087 ms</td> <td style="text-align: right;">2384 ms / 3972 ms</td> <td style="text-align: right;">−6% / −3%</td> </tr> </tbody> </table> <p class=nj-table-caption>Compile time with filesystem support switched off and <code>JSON_NO_IO</code> defined, against the default configuration, at <code>-O0</code> and <code>-O2</code>.</p> <p>The trade-off: the library's own <code>to_json</code>/<code>from_json</code> overloads for <code>std::filesystem::path</code> (which store the path as <abbr title="Unicode Transformation Format">UTF</abbr>-8) are removed. In a test with both compilers on macOS, paths still converted through their implicit conversion from and to <code>std::string</code>; whether that fallback exists depends on the platform's native path string type (<code>std::wstring</code> on Windows), so check code that converts paths. <code>JSON_NO_IO</code> additionally removes all stream functions (see <a href=#options-without-effect-on-compile-times>Options without effect</a>).</p> <h2 id=switch-off-stdformat-support>Switch off <code>std::format</code> support<a class=headerlink href=#switch-off-stdformat-support title="Permanent link">¶</a></h2> <p>With C++20 and a standard library that provides <code><format></code>, the library includes it to provide a <a href=../../api/basic_json/std_formatter/ ><code>std::formatter<basic_json></code></a> specialization (see <a href=../../api/macros/json_has_std_format/ ><code>JSON_HAS_STD_FORMAT</code></a>). If you do not format <abbr title="JavaScript Object Notation">JSON</abbr> values with <code>std::format</code>, switch the detection off for the whole project:</p> <div class=highlight><pre><span></span><code><span class=nb>target_compile_definitions</span><span class=p>(</span><span class=s>my_target</span><span class=w> </span><span class=s>PRIVATE</span><span class=w> </span><span class=s>JSON_HAS_STD_FORMAT=0</span><span class=p>)</span>
|
||||
</code></pre></div> <table> <thead> <tr> <th>Compiler</th> <th>Translation unit</th> <th style="text-align: right;">Default (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;"><code>std::format</code> off (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;">Change</th> </tr> </thead> <tbody> <tr> <td>Apple clang</td> <td>model</td> <td style="text-align: right;">1283 ms / 1544 ms</td> <td style="text-align: right;">1269 ms / 1539 ms</td> <td style="text-align: right;">−1% / 0%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>model</td> <td style="text-align: right;">2026 ms / 2482 ms</td> <td style="text-align: right;">1944 ms / 2416 ms</td> <td style="text-align: right;">−4% / −3%</td> </tr> <tr> <td>Apple clang</td> <td>parsing</td> <td style="text-align: right;">1350 ms / 2278 ms</td> <td style="text-align: right;">1347 ms / 2300 ms</td> <td style="text-align: right;">0% / +1%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>parsing</td> <td style="text-align: right;">2715 ms / 4351 ms</td> <td style="text-align: right;">2644 ms / 4287 ms</td> <td style="text-align: right;">−3% / −1%</td> </tr> </tbody> </table> <p class=nj-table-caption>Compile time with <code>JSON_HAS_STD_FORMAT=0</code> against the default configuration, both with <code>-std=c++20</code>, at <code>-O0</code> and <code>-O2</code>.</p> <p>The saving is small, and only <abbr title="GNU Compiler Collection">GCC</abbr> shows it: libstdc++'s <code><format></code> is an additional header that nothing else in the translation unit includes, so the macro removes it (and the headers it pulls in). With libc++ (Apple clang), other standard headers include <code><format></code> anyway, so the macro only removes the library's own formatter, and the difference is within the noise.</p> <p>The trade-off: the <code>std::formatter<basic_json></code> specialization is removed, so <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>format</span><span class=p>(</span><span class=s>"{}"</span><span class=p>,</span><span class=w> </span><span class=n>j</span><span class=p>)</span></code>, and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>format_to</span></code> with a <abbr title="JavaScript Object Notation">JSON</abbr> value no longer compile (in a test with both compilers on macOS, the error was that <code>std::formatter</code> must be specialized for the type). Use <a href=../../api/basic_json/dump/ ><code>dump()</code></a>, or the stream operators, instead.</p> <h2 id=switch-off-ranges-support>Switch off ranges support<a class=headerlink href=#switch-off-ranges-support title="Permanent link">¶</a></h2> <p>With C++20 and a standard library that provides <code><ranges></code>, the library includes it to declare the <a href=../../api/basic_json/items/ ><code>items()</code></a> proxy a <em>borrowed range</em>, and uses the <code>std::ranges</code> concepts and algorithms for construction from range views and for iterator input (see <a href=../../api/macros/json_has_ranges/ ><code>JSON_HAS_RANGES</code></a>). If you do not need these, switch the detection off for the whole project:</p> <div class=highlight><pre><span></span><code><span class=nb>target_compile_definitions</span><span class=p>(</span><span class=s>my_target</span><span class=w> </span><span class=s>PRIVATE</span><span class=w> </span><span class=s>JSON_HAS_RANGES=0</span><span class=p>)</span>
|
||||
</code></pre></div> <table> <thead> <tr> <th>Compiler</th> <th>Translation unit</th> <th style="text-align: right;">Default (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;">Ranges off (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;">Change</th> </tr> </thead> <tbody> <tr> <td>Apple clang</td> <td>model</td> <td style="text-align: right;">1283 ms / 1544 ms</td> <td style="text-align: right;">1237 ms / 1495 ms</td> <td style="text-align: right;">−4% / −3%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>model</td> <td style="text-align: right;">2026 ms / 2482 ms</td> <td style="text-align: right;">2004 ms / 2464 ms</td> <td style="text-align: right;">−1% / −1%</td> </tr> <tr> <td>Apple clang</td> <td>parsing</td> <td style="text-align: right;">1350 ms / 2278 ms</td> <td style="text-align: right;">1311 ms / 2284 ms</td> <td style="text-align: right;">−3% / 0%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>parsing</td> <td style="text-align: right;">2715 ms / 4351 ms</td> <td style="text-align: right;">2705 ms / 4427 ms</td> <td style="text-align: right;">0% / +2%</td> </tr> </tbody> </table> <p class=nj-table-caption>Compile time with <code>JSON_HAS_RANGES=0</code> against the default configuration, both with <code>-std=c++20</code>, at <code>-O0</code> and <code>-O2</code>.</p> <p>The saving is small as well, and with <abbr title="GNU Compiler Collection">GCC</abbr> it is not distinguishable from the noise: its <code><format></code> includes <code><span></code>, so with <code>std::format</code> support left on, part of the ranges machinery is still parsed. Both macros together save more than either alone:</p> <div class=highlight><pre><span></span><code><span class=nb>target_compile_definitions</span><span class=p>(</span><span class=s>my_target</span><span class=w> </span><span class=s>PRIVATE</span><span class=w> </span><span class=s>JSON_HAS_STD_FORMAT=0</span><span class=w> </span><span class=s>JSON_HAS_RANGES=0</span><span class=p>)</span>
|
||||
</code></pre></div> <table> <thead> <tr> <th>Compiler</th> <th>Translation unit</th> <th style="text-align: right;">Default (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;">Both off (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;">Change</th> </tr> </thead> <tbody> <tr> <td>Apple clang</td> <td>model</td> <td style="text-align: right;">1283 ms / 1544 ms</td> <td style="text-align: right;">1223 ms / 1504 ms</td> <td style="text-align: right;">−5% / −3%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>model</td> <td style="text-align: right;">2026 ms / 2482 ms</td> <td style="text-align: right;">1914 ms / 2378 ms</td> <td style="text-align: right;">−6% / −4%</td> </tr> <tr> <td>Apple clang</td> <td>parsing</td> <td style="text-align: right;">1350 ms / 2278 ms</td> <td style="text-align: right;">1311 ms / 2271 ms</td> <td style="text-align: right;">−3% / 0%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>parsing</td> <td style="text-align: right;">2715 ms / 4351 ms</td> <td style="text-align: right;">2635 ms / 4260 ms</td> <td style="text-align: right;">−3% / −2%</td> </tr> </tbody> </table> <p class=nj-table-caption>Compile time with <code>JSON_HAS_STD_FORMAT=0</code> and <code>JSON_HAS_RANGES=0</code> against the default configuration, both with <code>-std=c++20</code>, at <code>-O0</code> and <code>-O2</code>.</p> <p>The trade-offs of <code>JSON_HAS_RANGES=0</code> are larger than those of the other options on this page:</p> <ul> <li><code>items()</code> is no longer a borrowed range. On a temporary, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>ranges</span><span class=o>::</span><span class=n>begin</span><span class=p>(</span><span class=n>make_json</span><span class=p>().</span><span class=n>items</span><span class=p>())</span></code> no longer compiles, and <code>std::ranges</code> algorithms such as <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>ranges</span><span class=o>::</span><span class=n>find_if</span><span class=p>(</span><span class=n>make_json</span><span class=p>().</span><span class=n>items</span><span class=p>(),</span><span class=w> </span><span class=n>pred</span><span class=p>)</span></code> return <code>std::ranges::dangling</code> instead of an iterator, so code that uses the result no longer compiles. Calling them on <code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>items</span><span class=p>()</span></code> for a named <code>j</code> is not affected.</li> <li><a href=../../api/macros/json_has_range_view_conversion/ ><code>JSON_HAS_RANGE_VIEW_CONVERSION</code></a> becomes <code>0</code>, so a <abbr title="JavaScript Object Notation">JSON</abbr> array can no longer be constructed from a <code>std::ranges::view</code> such as the result of <code>std::views::filter</code>. The assignment <code class=highlight><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>|</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>views</span><span class=o>::</span><span class=n>filter</span><span class=p>(</span><span class=n>pred</span><span class=p>);</span></code> no longer compiles.</li> <li>Parsing from an iterator pair loses the C++20 fast path for contiguous iterators that are not pointers (for example those of <code>std::vector</code>) and falls back to byte-at-a-time scanning. The result is the same; parsing may be slower (not measured here).</li> </ul> <p>The first two were checked with both compilers on macOS; the third is read from the library's source.</p> <h2 id=use-c20-modules>Use C++20 modules<a class=headerlink href=#use-c20-modules title="Permanent link">¶</a></h2> <p>With a toolchain that supports named modules, <code>import nlohmann.json;</code> compiles the library once into a module and avoids parsing the header in every translation unit. See <a href=../../features/modules/ >Modules</a> for requirements and known issues. Module support is experimental and currently depends heavily on the compiler version.</p> <div class=nj-fig-wrap> <p><svg class="nj-fig nj-charts nj-charts--compile" viewbox="0 0 440 282" role=img aria-labelledby="nj-charts-mod nj-charts-mod-d" xmlns=http://www.w3.org/2000/svg> <title id=nj-charts-mod>Compile time per translation unit with import nlohmann.json at -O0, and the one-off cost of the module</title> <desc id=nj-charts-mod-d>Horizontal bars, negative is faster, medians of nine runs, -std=c++20 for both. model TU, Apple clang: -48 % (1311 ms to 680 ms); model TU, G​CC 16: -47 % (2156 ms to 1153 ms); parsing TU, Apple clang: -42 % (1387 ms to 800 ms); parsing TU, G​CC 16: -36 % (2838 ms to 1810 ms); one-off build, Apple clang: 1630 ms; oLine truncated
|
||||
clang++<span class=w> </span>-std<span class=o>=</span>c++20<span class=w> </span>-fcxx-modules<span class=w> </span>-O0<span class=w> </span>-I<span class=w> </span>single_include<span class=w> </span>-c<span class=w> </span>-x<span class=w> </span>c++-module<span class=w> </span>src/modules/json.cppm<span class=w> </span><span class=se>\</span>
|
||||
<span class=w> </span>-fmodule-output<span class=o>=</span>nlohmann.json.pcm<span class=w> </span>-o<span class=w> </span>json_mod.o
|
||||
clang++<span class=w> </span>-std<span class=o>=</span>c++20<span class=w> </span>-fcxx-modules<span class=w> </span>-O0<span class=w> </span>-fmodule-file<span class=o>=</span>nlohmann.json<span class=o>=</span>nlohmann.json.pcm<span class=w> </span>-c<span class=w> </span>model.cpp
|
||||
@@ -70,4 +73,4 @@ clang++<span class=w> </span>-std<span class=o>=</span>c++17<span class=w> </spa
|
||||
<span class=c1># GCC 16 (Homebrew on macOS): pch.hpp.gch next to pch.hpp is picked up by -include pch.hpp</span>
|
||||
g++-16<span class=w> </span>--sysroot<span class=o>=</span><span class=k>$(</span>xcrun<span class=w> </span>--show-sdk-path<span class=k>)</span><span class=w> </span>-std<span class=o>=</span>c++17<span class=w> </span>-O0<span class=w> </span>-I<span class=w> </span>single_include<span class=w> </span>-x<span class=w> </span>c++-header<span class=w> </span>pch.hpp<span class=w> </span>-o<span class=w> </span>pch.hpp.gch
|
||||
g++-16<span class=w> </span>--sysroot<span class=o>=</span><span class=k>$(</span>xcrun<span class=w> </span>--show-sdk-path<span class=k>)</span><span class=w> </span>-std<span class=o>=</span>c++17<span class=w> </span>-O0<span class=w> </span>-I<span class=w> </span>single_include<span class=w> </span>-include<span class=w> </span>pch.hpp<span class=w> </span>-Winvalid-pch<span class=w> </span>-c<span class=w> </span>model.cpp
|
||||
</code></pre></div> </details> <h2 id=options-without-effect-on-compile-times>Options without effect on compile times<a class=headerlink href=#options-without-effect-on-compile-times title="Permanent link">¶</a></h2> <p>Some configuration macros change what the library declares, but do not measurably change compile times:</p> <table> <thead> <tr> <th>Macro</th> <th style="text-align: right;">Apple clang, model (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;"><abbr title="GNU Compiler Collection">GCC</abbr> 16, model (<code>-O0</code> / <code>-O2</code>)</th> </tr> </thead> <tbody> <tr> <td>default</td> <td style="text-align: right;">1051 ms / 1300 ms</td> <td style="text-align: right;">1801 ms / 2106 ms</td> </tr> <tr> <td><a href=../../api/macros/json_no_io/ ><code>JSON_NO_IO</code></a></td> <td style="text-align: right;">1034 ms / 1288 ms</td> <td style="text-align: right;">1756 ms / 2087 ms</td> </tr> <tr> <td><a href=../../api/macros/json_use_global_udls/ ><code>JSON_USE_GLOBAL_UDLS</code></a><code>=0</code></td> <td style="text-align: right;">1055 ms / 1298 ms</td> <td style="text-align: right;">1783 ms / 2109 ms</td> </tr> </tbody> </table> <p class=nj-table-caption>Compile time of the model translation unit with each macro defined; the first row is the default configuration for comparison.</p> <p><code>JSON_NO_IO</code> removes the library's stream functions and its <code>#include</code>s of <code><istream></code> and <code><ostream></code>, but with the default C++17 settings <code><filesystem></code> includes these headers anyway (and with libc++, <code><vector></code> already includes <code><streambuf></code> via <code><locale></code>), so only a few hundred lines of the library's own stream code go away. It also does not change what the translation unit instantiates. On the parsing translation unit, the change is 0% / 0% (Apple clang) and −2% / 0% (<abbr title="GNU Compiler Collection">GCC</abbr>). To get a saving, combine it with <a href=#switch-off-filesystem-support>switching off filesystem support</a>.</p> <p><code>JSON_USE_GLOBAL_UDLS</code> only controls <em>where</em> the literals are declared; to avoid their cost, use <code>JSON_NO_AUTOMATIC_UDLS</code> instead.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/macros/json_has_filesystem/ ><code>JSON_HAS_FILESYSTEM</code></a> - control the conversions from and to <code>std::filesystem::path</code></li> <li><a href=../../api/macros/json_no_automatic_udls/ ><code>JSON_NO_AUTOMATIC_UDLS</code></a> - do not include the user-defined string literals automatically</li> <li><a href=../../features/modules/ >Modules</a> - C++20 module support</li> <li><a href=../ >Header only</a> - including the library</li> <li><a href=../../api/macros/ >Macros</a> - reference of all macros</li> <li><a href=../../features/macros/ >Supported Macros</a> - overview of all configuration macros</li> <li><a href=../../features/performance/ >Performance</a> - run-time performance</li> <li><a href=../../api/macros/json_no_io/ ><code>JSON_NO_IO</code></a> - remove all stream and file I/O</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../pkg-config/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Pkg-config"> <div class="md-footer__button md-icon"> <svLine truncated
|
||||
</code></pre></div> </details> <h2 id=options-without-effect-on-compile-times>Options without effect on compile times<a class=headerlink href=#options-without-effect-on-compile-times title="Permanent link">¶</a></h2> <p>Some configuration macros change what the library declares, but do not measurably change compile times:</p> <table> <thead> <tr> <th>Macro</th> <th style="text-align: right;">Apple clang, model (<code>-O0</code> / <code>-O2</code>)</th> <th style="text-align: right;"><abbr title="GNU Compiler Collection">GCC</abbr> 16, model (<code>-O0</code> / <code>-O2</code>)</th> </tr> </thead> <tbody> <tr> <td>default</td> <td style="text-align: right;">1051 ms / 1300 ms</td> <td style="text-align: right;">1801 ms / 2106 ms</td> </tr> <tr> <td><a href=../../api/macros/json_no_io/ ><code>JSON_NO_IO</code></a></td> <td style="text-align: right;">1034 ms / 1288 ms</td> <td style="text-align: right;">1756 ms / 2087 ms</td> </tr> <tr> <td><a href=../../api/macros/json_use_global_udls/ ><code>JSON_USE_GLOBAL_UDLS</code></a><code>=0</code></td> <td style="text-align: right;">1055 ms / 1298 ms</td> <td style="text-align: right;">1783 ms / 2109 ms</td> </tr> </tbody> </table> <p class=nj-table-caption>Compile time of the model translation unit with each macro defined; the first row is the default configuration for comparison.</p> <p><code>JSON_NO_IO</code> removes the library's stream functions and its <code>#include</code>s of <code><istream></code> and <code><ostream></code>, but with the default C++17 settings <code><filesystem></code> includes these headers anyway (and with libc++, <code><vector></code> already includes <code><streambuf></code> via <code><locale></code>), so only a few hundred lines of the library's own stream code go away. It also does not change what the translation unit instantiates. On the parsing translation unit, the change is 0% / 0% (Apple clang) and −2% / 0% (<abbr title="GNU Compiler Collection">GCC</abbr>). To get a saving, combine it with <a href=#switch-off-filesystem-support>switching off filesystem support</a>.</p> <p><code>JSON_USE_GLOBAL_UDLS</code> only controls <em>where</em> the literals are declared; to avoid their cost, use <code>JSON_NO_AUTOMATIC_UDLS</code> instead.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/macros/json_has_filesystem/ ><code>JSON_HAS_FILESYSTEM</code></a> - control the conversions from and to <code>std::filesystem::path</code></li> <li><a href=../../api/macros/json_has_std_format/ ><code>JSON_HAS_STD_FORMAT</code></a> - control the <code>std::formatter</code> specialization</li> <li><a href=../../api/macros/json_has_ranges/ ><code>JSON_HAS_RANGES</code></a> - control the use of <code>std::ranges</code></li> <li><a href=../../api/macros/json_no_automatic_udls/ ><code>JSON_NO_AUTOMATIC_UDLS</code></a> - do not include the user-defined string literals automatically</li> <li><a href=../../features/modules/ >Modules</a> - C++20 module support</li> <li><a href=../ >Header only</a> - including the library</li> <li><a href=../../api/macros/ >Macros</a> - reference of all macros</li> <li><a href=../../features/macros/ >Supported Macros</a> - overview of all configuration macros</li> <li><a href=../../features/performance/ >Performance</a> - run-time performance</li> <li><a href=../../api/macros/json_no_io/ ><code>JSON_NO_IO</code></a> - remove all stream and file I/O</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="November 5, 2026 15:04:39 UTC">November 5, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.4Line truncated
|
||||
@@ -6,6 +6,8 @@ Measurements
|
||||
|
||||
The numbers below are medians of the wall-clock time of nine runs compiling a single translation unit with `-std=c++17 -c` against the single-header version, with Apple clang 21.0.0 (clang-2100.3.34.2) and Homebrew GCC 16.2.0 on macOS (Apple M1 Max). All configurations were run interleaved, in alternating order. They show the order of magnitude to expect; measure your own code before and after a change.
|
||||
|
||||
The two sections on `std::format` and ranges apply to C++20 only and were measured with `-std=c++20`, so their baselines are slower than the others. Their differences are mostly within the run-to-run noise of a few percent: two complete runs of the same configuration differed by up to 10% in the median. The tables show the later run (one outlier cell was repeated).
|
||||
|
||||
The translation units are listed below. They replace the ones used for earlier versions of this page, whose sources were not published, so the numbers differ from earlier versions.
|
||||
|
||||
Measured translation units
|
||||
@@ -61,7 +63,7 @@ void to_json(nlohmann::json& j, const person& p);
|
||||
void from_json(const nlohmann::json& j, person& p);
|
||||
```
|
||||
|
||||
Change in compile time at `-O0` against the default configuration of the same translation unit, taken from the tables below (medians of nine runs, see above). * computed from the milliseconds in [Options without effect on compile times](#options-without-effect-on-compile-times).
|
||||
Change in compile time at `-O0` against the default configuration of the same translation unit, taken from the tables below (medians of nine runs, see above; the `std::format` and ranges rows are measured with `-std=c++20`). * computed from the milliseconds in [Options without effect on compile times](#options-without-effect-on-compile-times).
|
||||
|
||||
## Include `json_fwd.hpp` in headers
|
||||
|
||||
@@ -192,6 +194,67 @@ Compile time with filesystem support switched off and `JSON_NO_IO` defined, agai
|
||||
|
||||
The trade-off: the library's own `to_json`/`from_json` overloads for `std::filesystem::path` (which store the path as UTF-8) are removed. In a test with both compilers on macOS, paths still converted through their implicit conversion from and to `std::string`; whether that fallback exists depends on the platform's native path string type (`std::wstring` on Windows), so check code that converts paths. `JSON_NO_IO` additionally removes all stream functions (see [Options without effect](#options-without-effect-on-compile-times)).
|
||||
|
||||
## Switch off `std::format` support
|
||||
|
||||
With C++20 and a standard library that provides `<format>`, the library includes it to provide a [`std::formatter<basic_json>`](https://json.nlohmann.me/api/basic_json/std_formatter/index.md) specialization (see [`JSON_HAS_STD_FORMAT`](https://json.nlohmann.me/api/macros/json_has_std_format/index.md)). If you do not format JSON values with `std::format`, switch the detection off for the whole project:
|
||||
|
||||
```
|
||||
target_compile_definitions(my_target PRIVATE JSON_HAS_STD_FORMAT=0)
|
||||
```
|
||||
|
||||
| Compiler | Translation unit | Default (`-O0` / `-O2`) | `std::format` off (`-O0` / `-O2`) | Change |
|
||||
| ----------- | ---------------- | ----------------------- | --------------------------------- | --------- |
|
||||
| Apple clang | model | 1283 ms / 1544 ms | 1269 ms / 1539 ms | −1% / 0% |
|
||||
| GCC 16 | model | 2026 ms / 2482 ms | 1944 ms / 2416 ms | −4% / −3% |
|
||||
| Apple clang | parsing | 1350 ms / 2278 ms | 1347 ms / 2300 ms | 0% / +1% |
|
||||
| GCC 16 | parsing | 2715 ms / 4351 ms | 2644 ms / 4287 ms | −3% / −1% |
|
||||
|
||||
Compile time with `JSON_HAS_STD_FORMAT=0` against the default configuration, both with `-std=c++20`, at `-O0` and `-O2`.
|
||||
|
||||
The saving is small, and only GCC shows it: libstdc++'s `<format>` is an additional header that nothing else in the translation unit includes, so the macro removes it (and the headers it pulls in). With libc++ (Apple clang), other standard headers include `<format>` anyway, so the macro only removes the library's own formatter, and the difference is within the noise.
|
||||
|
||||
The trade-off: the `std::formatter<basic_json>` specialization is removed, so `std::format("{}", j)`, and `std::format_to` with a JSON value no longer compile (in a test with both compilers on macOS, the error was that `std::formatter` must be specialized for the type). Use [`dump()`](https://json.nlohmann.me/api/basic_json/dump/index.md), or the stream operators, instead.
|
||||
|
||||
## Switch off ranges support
|
||||
|
||||
With C++20 and a standard library that provides `<ranges>`, the library includes it to declare the [`items()`](https://json.nlohmann.me/api/basic_json/items/index.md) proxy a *borrowed range*, and uses the `std::ranges` concepts and algorithms for construction from range views and for iterator input (see [`JSON_HAS_RANGES`](https://json.nlohmann.me/api/macros/json_has_ranges/index.md)). If you do not need these, switch the detection off for the whole project:
|
||||
|
||||
```
|
||||
target_compile_definitions(my_target PRIVATE JSON_HAS_RANGES=0)
|
||||
```
|
||||
|
||||
| Compiler | Translation unit | Default (`-O0` / `-O2`) | Ranges off (`-O0` / `-O2`) | Change |
|
||||
| ----------- | ---------------- | ----------------------- | -------------------------- | --------- |
|
||||
| Apple clang | model | 1283 ms / 1544 ms | 1237 ms / 1495 ms | −4% / −3% |
|
||||
| GCC 16 | model | 2026 ms / 2482 ms | 2004 ms / 2464 ms | −1% / −1% |
|
||||
| Apple clang | parsing | 1350 ms / 2278 ms | 1311 ms / 2284 ms | −3% / 0% |
|
||||
| GCC 16 | parsing | 2715 ms / 4351 ms | 2705 ms / 4427 ms | 0% / +2% |
|
||||
|
||||
Compile time with `JSON_HAS_RANGES=0` against the default configuration, both with `-std=c++20`, at `-O0` and `-O2`.
|
||||
|
||||
The saving is small as well, and with GCC it is not distinguishable from the noise: its `<format>` includes `<span>`, so with `std::format` support left on, part of the ranges machinery is still parsed. Both macros together save more than either alone:
|
||||
|
||||
```
|
||||
target_compile_definitions(my_target PRIVATE JSON_HAS_STD_FORMAT=0 JSON_HAS_RANGES=0)
|
||||
```
|
||||
|
||||
| Compiler | Translation unit | Default (`-O0` / `-O2`) | Both off (`-O0` / `-O2`) | Change |
|
||||
| ----------- | ---------------- | ----------------------- | ------------------------ | --------- |
|
||||
| Apple clang | model | 1283 ms / 1544 ms | 1223 ms / 1504 ms | −5% / −3% |
|
||||
| GCC 16 | model | 2026 ms / 2482 ms | 1914 ms / 2378 ms | −6% / −4% |
|
||||
| Apple clang | parsing | 1350 ms / 2278 ms | 1311 ms / 2271 ms | −3% / 0% |
|
||||
| GCC 16 | parsing | 2715 ms / 4351 ms | 2635 ms / 4260 ms | −3% / −2% |
|
||||
|
||||
Compile time with `JSON_HAS_STD_FORMAT=0` and `JSON_HAS_RANGES=0` against the default configuration, both with `-std=c++20`, at `-O0` and `-O2`.
|
||||
|
||||
The trade-offs of `JSON_HAS_RANGES=0` are larger than those of the other options on this page:
|
||||
|
||||
- `items()` is no longer a borrowed range. On a temporary, `std::ranges::begin(make_json().items())` no longer compiles, and `std::ranges` algorithms such as `std::ranges::find_if(make_json().items(), pred)` return `std::ranges::dangling` instead of an iterator, so code that uses the result no longer compiles. Calling them on `j.items()` for a named `j` is not affected.
|
||||
- [`JSON_HAS_RANGE_VIEW_CONVERSION`](https://json.nlohmann.me/api/macros/json_has_range_view_conversion/index.md) becomes `0`, so a JSON array can no longer be constructed from a `std::ranges::view` such as the result of `std::views::filter`. The assignment `json j = v | std::views::filter(pred);` no longer compiles.
|
||||
- Parsing from an iterator pair loses the C++20 fast path for contiguous iterators that are not pointers (for example those of `std::vector`) and falls back to byte-at-a-time scanning. The result is the same; parsing may be slower (not measured here).
|
||||
|
||||
The first two were checked with both compilers on macOS; the third is read from the library's source.
|
||||
|
||||
## Use C++20 modules
|
||||
|
||||
With a toolchain that supports named modules, `import nlohmann.json;` compiles the library once into a module and avoids parsing the header in every translation unit. See [Modules](https://json.nlohmann.me/features/modules/index.md) for requirements and known issues. Module support is experimental and currently depends heavily on the compiler version.
|
||||
@@ -290,6 +353,8 @@ Compile time of the model translation unit with each macro defined; the first ro
|
||||
## See also
|
||||
|
||||
- [`JSON_HAS_FILESYSTEM`](https://json.nlohmann.me/api/macros/json_has_filesystem/index.md) - control the conversions from and to `std::filesystem::path`
|
||||
- [`JSON_HAS_STD_FORMAT`](https://json.nlohmann.me/api/macros/json_has_std_format/index.md) - control the `std::formatter` specialization
|
||||
- [`JSON_HAS_RANGES`](https://json.nlohmann.me/api/macros/json_has_ranges/index.md) - control the use of `std::ranges`
|
||||
- [`JSON_NO_AUTOMATIC_UDLS`](https://json.nlohmann.me/api/macros/json_no_automatic_udls/index.md) - do not include the user-defined string literals automatically
|
||||
- [Modules](https://json.nlohmann.me/features/modules/index.md) - C++20 module support
|
||||
- [Header only](https://json.nlohmann.me/integration/index.md) - including the library
|
||||
|
||||
@@ -1 +1 @@
|
||||
{"config":{"lang":["en"],"separator":"[\\s\\-\\.]","pipeline":["stopWordFilter"],"fields":{"title":{"boost":1000.0},"text":{"boost":1.0},"tags":{"boost":1000000.0}}},"docs":[{"location":"","title":"JSON for Modern C++","text":"<ul> <li> <p> Features</p> <p>Creating, parsing, accessing, and serializing JSON values, converting your own C++ types, JSON Pointer/Patch/Merge Patch, binary formats, zero-copy views, and more.</p> <p> Features</p> </li> <li> <p> Integration</p> <p>Add the library to your project via a single header, CMake, a package manager, or pkg-config.</p> <p> Integration</p> </li> <li> <p> API documentation</p> <p>The complete reference for <code>basic_json</code> and its member functions, types, and related classes, as well as for <code>json_document</code> and <code>json_view</code>.</p> <p> API documentation</p> </li> <li> <p> FAQ</p> <p>Answers to common questions and known surprises when using the library.</p> <p> FAQ</p> </li> <li> <p> Releases</p> <p>What changed in each release, with links to the relevant documentation.</p> <p> Releases</p> </li> <li> <p> Community</p> <p>The ecosystem, contribution guidelines, governance, and quality assurance around the project.</p> <p> Community</p> </li> </ul> <p> </p>"},{"location":"api/is_json_view/","title":"nlohmann::is_json_view","text":"<p>Defined in header <code><nlohmann/json_view.hpp></code></p> <pre><code>template<typename T>\nstruct is_json_view;\n</code></pre> <p>A type trait: <code>is_json_view<T>::value</code> is <code>true</code> if <code>T</code> is a <code>basic_json_view</code> of any <code>BasicJsonType</code> and any <code>view_mode</code> -- <code>json_view</code>, <code>json_editable_view</code>, <code>ordered_json_view</code>, ... -- and <code>false</code> otherwise, for <code>basic_json</code> as well.</p> <p>It is meant for constraining templates that accept views, in particular a <code>from_json()</code> that should read views directly: <code>get<T>()</code> on a view calls a <code>from_json()</code> with the view only if that <code>from_json()</code> accepts views but not arbitrary types.</p>"},{"location":"api/is_json_view/#template-parameters","title":"Template parameters","text":"<code>T</code> the type to check (without cv-qualifiers or references)"},{"location":"api/is_json_view/#notes","title":"Notes","text":"<p>A view is recognized by its member type <code>basic_json_type</code> (a <code>basic_json</code> specialization) and its <code>materialize()</code> returning that type, so the trait needs no declaration of the view class.</p>"},{"location":"api/is_json_view/#examples","title":"Examples","text":"Example <p>The example below shows the trait, and a <code>from_json()</code> constrained to <code>json</code> values and views: <code>get<T>()</code> on a view reads the view directly.</p> <pre><code>#include <iostream>\n#include <type_traits>\n#include <nlohmann/json_view.hpp>\n\nusing nlohmann::json;\n\nnamespace ns\n{\nstruct person\n{\n std::string name;\n int age = 0;\n};\n\n// reads json values and views (of any document type), but nothing else\ntemplate < typename J, typename std::enable_if < nlohmann::is_json_view<J>::value || std::is_same<J, json>::value, int >::type = 0 >\nvoid from_json(const J& j, person& p)\n{\n p.name = j.at(\"name\").template get<std::string>();\n p.age = j.at(\"age\").template get<int>();\n}\n} // namespace ns\n\nint main()\n{\n std::cout << std::boolalpha\n << nlohmann::is_json_view<nlohmann::json_view>::value << ' '\n << nlohmann::is_json_view<nlohmann::ordered_json_editable_view>::value << ' '\n << nlohmann::is_json_view<json>::value << '\\n';\n\n const nlohmann::json_document doc = nlohmann::json_document::parse(R\"({\"name\": \"Ada\", \"age\": 36})\");\n const auto p = doc.root().get<ns::person>(); // reads the view, no json value is built\n std::cout << p.name << ' ' << p.age << '\\n';\n}\n</code></pre> <p>Output:</p> <pre><code>true true false\nAda 36\n</code></pre>"},{"location":"api/is_json_view/#see-also","title":"See also","text":"<ul> <li>basic_json_view - a handle to one value of a document</li> <li>get - which conversions read the view directly</li> <li>view_mode - read-only or editable</li> <li>Writing generic code - functions and <code>from_json()</code> for views</li> </ul>"},{"location":"api/is_json_view/#version-history","title":"Version history","text":"<p>Added in version 3.13.0 unreleased.</p>"},{"location":"api/json/","title":"nlohmann::json","text":"<pre><code>using json = basic_json<>;\n</code></pre> <p>This type is the default specialization of the basic_json class which uses the standard template types.</p>"},{"location":"api/json/#examples","title":"Examples","text":"Example <p>The example below demonstrates how Line truncated
|
||||
{"config":{"lang":["en"],"separator":"[\\s\\-\\.]","pipeline":["stopWordFilter"],"fields":{"title":{"boost":1000.0},"text":{"boost":1.0},"tags":{"boost":1000000.0}}},"docs":[{"location":"","title":"JSON for Modern C++","text":"<ul> <li> <p> Features</p> <p>Creating, parsing, accessing, and serializing JSON values, converting your own C++ types, JSON Pointer/Patch/Merge Patch, binary formats, zero-copy views, and more.</p> <p> Features</p> </li> <li> <p> Integration</p> <p>Add the library to your project via a single header, CMake, a package manager, or pkg-config.</p> <p> Integration</p> </li> <li> <p> API documentation</p> <p>The complete reference for <code>basic_json</code> and its member functions, types, and related classes, as well as for <code>json_document</code> and <code>json_view</code>.</p> <p> API documentation</p> </li> <li> <p> FAQ</p> <p>Answers to common questions and known surprises when using the library.</p> <p> FAQ</p> </li> <li> <p> Releases</p> <p>What changed in each release, with links to the relevant documentation.</p> <p> Releases</p> </li> <li> <p> Community</p> <p>The ecosystem, contribution guidelines, governance, and quality assurance around the project.</p> <p> Community</p> </li> </ul> <p> </p>"},{"location":"api/is_json_view/","title":"nlohmann::is_json_view","text":"<p>Defined in header <code><nlohmann/json_view.hpp></code></p> <pre><code>template<typename T>\nstruct is_json_view;\n</code></pre> <p>A type trait: <code>is_json_view<T>::value</code> is <code>true</code> if <code>T</code> is a <code>basic_json_view</code> of any <code>BasicJsonType</code> and any <code>view_mode</code> -- <code>json_view</code>, <code>json_editable_view</code>, <code>ordered_json_view</code>, ... -- and <code>false</code> otherwise, for <code>basic_json</code> as well.</p> <p>It is meant for constraining templates that accept views, in particular a <code>from_json()</code> that should read views directly: <code>get<T>()</code> on a view calls a <code>from_json()</code> with the view only if that <code>from_json()</code> accepts views but not arbitrary types.</p>"},{"location":"api/is_json_view/#template-parameters","title":"Template parameters","text":"<code>T</code> the type to check (without cv-qualifiers or references)"},{"location":"api/is_json_view/#notes","title":"Notes","text":"<p>A view is recognized by its member type <code>basic_json_type</code> (a <code>basic_json</code> specialization) and its <code>materialize()</code> returning that type, so the trait needs no declaration of the view class.</p>"},{"location":"api/is_json_view/#examples","title":"Examples","text":"Example <p>The example below shows the trait, and a <code>from_json()</code> constrained to <code>json</code> values and views: <code>get<T>()</code> on a view reads the view directly.</p> <pre><code>#include <iostream>\n#include <type_traits>\n#include <nlohmann/json_view.hpp>\n\nusing nlohmann::json;\n\nnamespace ns\n{\nstruct person\n{\n std::string name;\n int age = 0;\n};\n\n// reads json values and views (of any document type), but nothing else\ntemplate < typename J, typename std::enable_if < nlohmann::is_json_view<J>::value || std::is_same<J, json>::value, int >::type = 0 >\nvoid from_json(const J& j, person& p)\n{\n p.name = j.at(\"name\").template get<std::string>();\n p.age = j.at(\"age\").template get<int>();\n}\n} // namespace ns\n\nint main()\n{\n std::cout << std::boolalpha\n << nlohmann::is_json_view<nlohmann::json_view>::value << ' '\n << nlohmann::is_json_view<nlohmann::ordered_json_editable_view>::value << ' '\n << nlohmann::is_json_view<json>::value << '\\n';\n\n const nlohmann::json_document doc = nlohmann::json_document::parse(R\"({\"name\": \"Ada\", \"age\": 36})\");\n const auto p = doc.root().get<ns::person>(); // reads the view, no json value is built\n std::cout << p.name << ' ' << p.age << '\\n';\n}\n</code></pre> <p>Output:</p> <pre><code>true true false\nAda 36\n</code></pre>"},{"location":"api/is_json_view/#see-also","title":"See also","text":"<ul> <li>basic_json_view - a handle to one value of a document</li> <li>get - which conversions read the view directly</li> <li>view_mode - read-only or editable</li> <li>Writing generic code - functions and <code>from_json()</code> for views</li> </ul>"},{"location":"api/is_json_view/#version-history","title":"Version history","text":"<p>Added in version 3.13.0 unreleased.</p>"},{"location":"api/json/","title":"nlohmann::json","text":"<pre><code>using json = basic_json<>;\n</code></pre> <p>This type is the default specialization of the basic_json class which uses the standard template types.</p>"},{"location":"api/json/#examples","title":"Examples","text":"Example <p>The example below demonstrates how Line truncated
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