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@@ -212,6 +212,11 @@ Use the non-amalgamated version of the library. This option is `ON` by default.
Treat the library headers like system headers (i.e., adding `SYSTEM` to the [`target_include_directories`](https://cmake.org/cmake/help/latest/command/target_include_directories.html) call) to check for this library by tools like Clang-Tidy. This option is `OFF` by default.
### `JSON_StrictBinaryUTF8`
Check string values and object keys for valid UTF-8 in the CBOR, UBJSON, BJData, and BSON writers, by defining the
macro [`JSON_STRICT_BINARY_UTF8`](../api/macros/json_strict_binary_utf8.md). This option is `OFF` by default.
### `JSON_StrictNulHandling`
Reject a `'\0'` (NUL) byte in the input instead of treating it as end of input, by defining the macro
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<!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=author content="Niels Lohmann"><link href="https://json.nlohmann.me/integration/cmake/" rel="canonical"><link href=../migration_guide/ rel=prev><link href=../package_managers/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>CMake - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.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=indigo data-md-color-accent=indigo> <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=#cmake 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> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </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> CMake </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=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette 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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 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div class=md-search data-mdLine 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=author content="Niels Lohmann"><link href="https://json.nlohmann.me/integration/cmake/" rel="canonical"><link href=../migration_guide/ rel=prev><link href=../package_managers/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>CMake - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.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=indigo data-md-color-accent=indigo> <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=#cmake 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> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </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> CMake </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=indigo data-md-color-accent=indigo 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=indigo data-md-color-accent=indigo 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 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div class=md-search data-mdLine truncated
<span class=nb>project</span><span class=p>(</span><span class=s>ExampleProject</span><span class=w> </span><span class=s>LANGUAGES</span><span class=w> </span><span class=s>CXX</span><span class=p>)</span>
<span class=nb>find_package</span><span class=p>(</span><span class=s>nlohmann_json</span><span class=w> </span><span class=s>3.12.0</span><span class=w> </span><span class=s>REQUIRED</span><span class=p>)</span>
@@ -49,10 +49,10 @@
</code></pre></div> <p>However, the repository <a href="https://github.com/nlohmann/json">https://github.com/nlohmann/json</a> download size is quite large.</p> </div> <h2 id=cmake-options>CMake Options<a class=headerlink href=#cmake-options title="Permanent link">&para;</a></h2> <h3 id=json_buildtests><code>JSON_BuildTests</code><a class=headerlink href=#json_buildtests title="Permanent link">&para;</a></h3> <p>Build the unit tests when <a href="https://cmake.org/cmake/help/latest/command/enable_testing.html"><code>BUILD_TESTING</code></a> is enabled. This option is <code>ON</code> by default if the library's CMake project is the top project. That is, when integrating the library as described above, the test suite is not built unless explicitly switched on with this option.</p> <h3 id=json_ci><code>JSON_CI</code><a class=headerlink href=#json_ci title="Permanent link">&para;</a></h3> <p>Enable <abbr title="Continuous Integration">CI</abbr> build targets. The exact targets are used during the several <abbr title="Continuous Integration">CI</abbr> steps and are subject to change without notice. This option is <code>OFF</code> by default.</p> <h3 id=json_diagnostics><code>JSON_Diagnostics</code><a class=headerlink href=#json_diagnostics title="Permanent link">&para;</a></h3> <p>Enable <a href=../../home/exceptions/#extended-diagnostic-messages>extended diagnostic messages</a> by defining macro <a href=../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a>. This option is <code>OFF</code> by default.</p> <div class="admonition warning"> <p class=admonition-title>Does not apply to a pre-installed package</p> <p>This option only takes effect when building nlohmann/json from source as part of your own CMake project (e.g. via <a href=#fetchcontent><code>FetchContent</code></a> or <a href=#embedded><code>add_subdirectory</code></a>). It has <strong>no effect</strong> on a package that was already built and installed elsewhere (Homebrew, vcpkg, a system package, etc.) — the resulting compile definition is baked into the exported <code>nlohmann_jsonTargets.cmake</code> at install time, and <code>set(JSON_Diagnostics ON)</code> before <code>find_package()</code> does not change it (verified against the Homebrew-installed package: the exported target still carries a fixed <code>$&lt;$&lt;BOOL:OFF&gt;:JSON_DIAGNOSTICS=1&gt;</code>, regardless of any variable set in the consuming project).</p> <p>To enable extended diagnostics for a pre-installed package, override the imported target's property directly after <code>find_package()</code>:</p> <div class=highlight><pre><span></span><code><span class=nb>find_package</span><span class=p>(</span><span class=s>nlohmann_json</span><span class=w> </span><span class=s>REQUIRED</span><span class=p>)</span>
<span class=nb>set_target_properties</span><span class=p>(</span><span class=s>nlohmann_json::nlohmann_json</span><span class=w> </span><span class=s>PROPERTIES</span>
<span class=w> </span><span class=s>INTERFACE_COMPILE_DEFINITIONS</span><span class=w> </span><span class=s2>"JSON_DIAGNOSTICS=1"</span><span class=p>)</span>
</code></pre></div> <p>This only works cleanly when your project is the sole consumer of that imported target. If nlohmann_json is pulled in from more than one place in your dependency graph with different <code>JSON_DIAGNOSTICS</code> values, you may see a <code>"JSON_DIAGNOSTICS" redefined</code> compiler error, since conflicting <code>-D</code> flags can end up on the same compile command line.</p> </div> <h3 id=json_diagnostic_positions><code>JSON_Diagnostic_Positions</code><a class=headerlink href=#json_diagnostic_positions title="Permanent link">&para;</a></h3> <p>Enable position diagnostics by defining macro <a href=../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a>. This option is <code>OFF</code> by default.</p> <h3 id=json_disableenumserialization><code>JSON_DisableEnumSerialization</code><a class=headerlink href=#json_disableenumserialization title="Permanent link">&para;</a></h3> <p>Disable default <code>enum</code> serialization by defining the macro <a href=../../api/macros/json_disable_enum_serialization/ ><code>JSON_DISABLE_ENUM_SERIALIZATION</code></a>. This option is <code>OFF</code> by default.</p> <h3 id=json_disabletuplereferenceconversion><code>JSON_DisableTupleReferenceConversion</code><a class=headerlink href=#json_disabletuplereferenceconversion title="Permanent link">&para;</a></h3> <p>Disable the conversion from a one-element <code>std::tuple</code> holding a reference to a <abbr title="JavaScript Object Notation">JSON</abbr> value by defining the macro <a href=../../api/macros/json_disable_tuple_reference_conversion/ ><code>JSON_DISABLE_TUPLE_REFERENCE_CONVERSION</code></a>. This option is <code>OFF</code> by default.</p> <h3 id=json_fasttests><code>JSON_FastTests</code><a class=headerlink href=#json_fasttests title="Permanent link">&para;</a></h3> <p>Skip expensive/slow test suites. This option is <code>OFF</code> by default. Depends on <code>JSON_BuildTests</code>.</p> <h3 id=json_globaludls><code>JSON_GlobalUDLs</code><a class=headerlink href=#json_globaludls title="Permanent link">&para;</a></h3> <p>Place user-defined string literals in the global namespace by defining the macro <a href=../../api/macros/json_use_global_udls/ ><code>JSON_USE_GLOBAL_UDLS</code></a>. This option is <code>ON</code> by default; see the <a href=../migration_guide/#import-namespace-literals-for-udls>migration guide</a> for how to prepare code for the next major release, where the literals are removed from the global namespace.</p> <h3 id=json_implicitconversions><code>JSON_ImplicitConversions</code><a class=headerlink href=#json_implicitconversions title="Permanent link">&para;</a></h3> <p>Enable implicit conversions by defining macro <a href=../../api/macros/json_use_implicit_conversions/ ><code>JSON_USE_IMPLICIT_CONVERSIONS</code></a>. This option is <code>ON</code> by default; see the <a href=../migration_guide/#replace-implicit-conversions>migration guide</a> for how to prepare code for the next major release, where implicit conversions are switched off by default.</p> <h3 id=json_install><code>JSON_Install</code><a class=headerlink href=#json_install title="Permanent link">&para;</a></h3> <p>Install CMake targets during install step. This option is <code>ON</code> by default if the library's CMake project is the top project. Installing also generates a <a href=../pkg-config/ >pkg-config</a> file for tools that rely on <code>pkg-config</code> instead of CMake.</p> <h3 id=json_legacydiscardedvaluecomparison><code>JSON_LegacyDiscardedValueComparison</code><a class=headerlink href=#json_legacydiscardedvaluecomparison title="Permanent link">&para;</a></h3> <p>Enable the (incorrect) legacy comparison behavior of discarded <abbr title="JavaScript Object Notation">JSON</abbr> values by defining macro <a href=../../api/macros/json_use_legacy_discarded_value_comparison/ ><code>JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON</code></a>. This option is <code>OFF</code> by default.</p> <h3 id=json_multipleheaders><code>JSON_MultipleHeaders</code><a class=headerlink href=#json_multipleheaders title="Permanent link">&para;</a></h3> <p>Use the non-amalgamated version of the library. This option is <code>ON</code> by default.</p> <h3 id=json_systeminclude><code>JSON_SystemInclude</code><a class=headerlink href=#json_systeminclude title="Permanent link">&para;</a></h3> <p>Treat the library headers like system headers (i.e., adding <code>SYSTEM</code> to the <a href="https://cmake.org/cmake/help/latest/command/target_include_directories.html"><code>target_include_directories</code></a> call) to check for this library by tools like Clang-Tidy. This option is <code>OFF</code> by default.</p> <h3 id=json_strictnulhandling><code>JSON_StrictNulHandling</code><a class=headerlink href=#json_strictnulhandling title="Permanent link">&para;</a></h3> <p>Reject a <code>'\0'</code> (NUL) byte in the input instead of treating it as end of input, by defining the macro <a href=../../api/macros/jsLine truncated
</code></pre></div> <p>This only works cleanly when your project is the sole consumer of that imported target. If nlohmann_json is pulled in from more than one place in your dependency graph with different <code>JSON_DIAGNOSTICS</code> values, you may see a <code>"JSON_DIAGNOSTICS" redefined</code> compiler error, since conflicting <code>-D</code> flags can end up on the same compile command line.</p> </div> <h3 id=json_diagnostic_positions><code>JSON_Diagnostic_Positions</code><a class=headerlink href=#json_diagnostic_positions title="Permanent link">&para;</a></h3> <p>Enable position diagnostics by defining macro <a href=../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a>. This option is <code>OFF</code> by default.</p> <h3 id=json_disableenumserialization><code>JSON_DisableEnumSerialization</code><a class=headerlink href=#json_disableenumserialization title="Permanent link">&para;</a></h3> <p>Disable default <code>enum</code> serialization by defining the macro <a href=../../api/macros/json_disable_enum_serialization/ ><code>JSON_DISABLE_ENUM_SERIALIZATION</code></a>. This option is <code>OFF</code> by default.</p> <h3 id=json_disabletuplereferenceconversion><code>JSON_DisableTupleReferenceConversion</code><a class=headerlink href=#json_disabletuplereferenceconversion title="Permanent link">&para;</a></h3> <p>Disable the conversion from a one-element <code>std::tuple</code> holding a reference to a <abbr title="JavaScript Object Notation">JSON</abbr> value by defining the macro <a href=../../api/macros/json_disable_tuple_reference_conversion/ ><code>JSON_DISABLE_TUPLE_REFERENCE_CONVERSION</code></a>. This option is <code>OFF</code> by default.</p> <h3 id=json_fasttests><code>JSON_FastTests</code><a class=headerlink href=#json_fasttests title="Permanent link">&para;</a></h3> <p>Skip expensive/slow test suites. This option is <code>OFF</code> by default. Depends on <code>JSON_BuildTests</code>.</p> <h3 id=json_globaludls><code>JSON_GlobalUDLs</code><a class=headerlink href=#json_globaludls title="Permanent link">&para;</a></h3> <p>Place user-defined string literals in the global namespace by defining the macro <a href=../../api/macros/json_use_global_udls/ ><code>JSON_USE_GLOBAL_UDLS</code></a>. This option is <code>ON</code> by default; see the <a href=../migration_guide/#import-namespace-literals-for-udls>migration guide</a> for how to prepare code for the next major release, where the literals are removed from the global namespace.</p> <h3 id=json_implicitconversions><code>JSON_ImplicitConversions</code><a class=headerlink href=#json_implicitconversions title="Permanent link">&para;</a></h3> <p>Enable implicit conversions by defining macro <a href=../../api/macros/json_use_implicit_conversions/ ><code>JSON_USE_IMPLICIT_CONVERSIONS</code></a>. This option is <code>ON</code> by default; see the <a href=../migration_guide/#replace-implicit-conversions>migration guide</a> for how to prepare code for the next major release, where implicit conversions are switched off by default.</p> <h3 id=json_install><code>JSON_Install</code><a class=headerlink href=#json_install title="Permanent link">&para;</a></h3> <p>Install CMake targets during install step. This option is <code>ON</code> by default if the library's CMake project is the top project. Installing also generates a <a href=../pkg-config/ >pkg-config</a> file for tools that rely on <code>pkg-config</code> instead of CMake.</p> <h3 id=json_legacydiscardedvaluecomparison><code>JSON_LegacyDiscardedValueComparison</code><a class=headerlink href=#json_legacydiscardedvaluecomparison title="Permanent link">&para;</a></h3> <p>Enable the (incorrect) legacy comparison behavior of discarded <abbr title="JavaScript Object Notation">JSON</abbr> values by defining macro <a href=../../api/macros/json_use_legacy_discarded_value_comparison/ ><code>JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON</code></a>. This option is <code>OFF</code> by default.</p> <h3 id=json_multipleheaders><code>JSON_MultipleHeaders</code><a class=headerlink href=#json_multipleheaders title="Permanent link">&para;</a></h3> <p>Use the non-amalgamated version of the library. This option is <code>ON</code> by default.</p> <h3 id=json_systeminclude><code>JSON_SystemInclude</code><a class=headerlink href=#json_systeminclude title="Permanent link">&para;</a></h3> <p>Treat the library headers like system headers (i.e., adding <code>SYSTEM</code> to the <a href="https://cmake.org/cmake/help/latest/command/target_include_directories.html"><code>target_include_directories</code></a> call) to check for this library by tools like Clang-Tidy. This option is <code>OFF</code> by default.</p> <h3 id=json_strictbinaryutf8><code>JSON_StrictBinaryUTF8</code><a class=headerlink href=#json_strictbinaryutf8 title="Permanent link">&para;</a></h3> <p>Check string values and object keys for valid <abbr title="Unicode Transformation Format">UTF</abbr>-8 in the <abbr title="Concise Binary ObjecLine truncated
<span class=nb>add_subdirectory</span><span class=p>(</span><span class=s>path/to/json</span><span class=p>)</span>
<span class=nb>add_executable</span><span class=p>(</span><span class=s>myproject</span><span class=w> </span><span class=s>main.cpp</span><span class=p>)</span>
<span class=nb>target_link_libraries</span><span class=p>(</span><span class=s>myproject</span><span class=w> </span><span class=s>PRIVATE</span><span class=w> </span><span class=s>nlohmann_json_modules</span><span class=p>)</span>
<span class=nb>target_compile_definitions</span><span class=p>(</span><span class=s>myproject</span><span class=w> </span><span class=s>PRIVATE</span><span class=w> </span><span class=s>NLOHMANN_JSON_BUILD_MODULES</span><span class=p>)</span>
</code></pre></div> <!-- 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 2, 2026 09:32:15 UTC">October 2, 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> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> <!-- 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 4, 2026 10:13:48 UTC">October 4, 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> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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+4
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@@ -186,6 +186,10 @@ Use the non-amalgamated version of the library. This option is `ON` by default.
Treat the library headers like system headers (i.e., adding `SYSTEM` to the [`target_include_directories`](https://cmake.org/cmake/help/latest/command/target_include_directories.html) call) to check for this library by tools like Clang-Tidy. This option is `OFF` by default.
### `JSON_StrictBinaryUTF8`
Check string values and object keys for valid UTF-8 in the CBOR, UBJSON, BJData, and BSON writers, by defining the macro [`JSON_STRICT_BINARY_UTF8`](https://json.nlohmann.me/api/macros/json_strict_binary_utf8/index.md). This option is `OFF` by default.
### `JSON_StrictNulHandling`
Reject a `'\0'` (NUL) byte in the input instead of treating it as end of input, by defining the macro [`JSON_STRICT_NUL_HANDLING`](https://json.nlohmann.me/api/macros/json_strict_nul_handling/index.md). This option is `OFF` by default.
+149
View File
@@ -0,0 +1,149 @@
# Compile times
The library is header-only and makes heavy use of templates, so every translation unit that includes
`<nlohmann/json.hpp>` pays for parsing the header and instantiating what it uses. This page lists the options to reduce
that cost, ordered by how much they typically save.
!!! info "Measurements"
The numbers below are medians of nine runs compiling a single translation unit with `-std=c++17 -c` against the
single-header version, with Apple clang and GCC 16 on macOS (Apple silicon). They show the order of magnitude to
expect; measure your own code before and after a change.
## Include `json_fwd.hpp` in headers
Header files that only need to *name* the `json` type — for function declarations, members held by pointer or
reference, or friend declarations — can include `<nlohmann/json_fwd.hpp>` instead of `<nlohmann/json.hpp>`. It only
forward-declares `basic_json`, `json`, `ordered_json`, `json_pointer`, and `adl_serializer`. The translation units that
actually use the values then include `<nlohmann/json.hpp>`.
```cpp title="person.hpp"
#pragma once
#include <nlohmann/json_fwd.hpp>
struct person;
void to_json(nlohmann::json& j, const person& p);
void from_json(const nlohmann::json& j, person& p);
```
```cpp title="person.cpp"
#include "person.hpp"
#include <nlohmann/json.hpp>
void to_json(nlohmann::json& j, const person& p) { /* ... */ }
void from_json(const nlohmann::json& j, person& p) { /* ... */ }
```
| Compiler | `json.hpp` (`-O0`) | `json_fwd.hpp` (`-O0`) | Change |
|-------------|-------------------:|-----------------------:|-------:|
| Apple clang | 704 ms | 329 ms | −53% |
| GCC 16 | 779 ms | 242 ms | −69% |
This is the most effective option, because it avoids the full header in every translation unit that includes
*your* headers.
## Opt out of the automatic user-defined string literals
The user-defined string literals [`operator""_json`](../api/operator_literal_json.md) and
[`operator""_json_pointer`](../api/operator_literal_json_pointer.md) are ordinary inline functions whose bodies call the
parser. As `<nlohmann/json.hpp>` includes them by default, every translation unit instantiates the parser, even if it
never parses anything itself.
Define [`JSON_NO_AUTOMATIC_UDLS`](../api/macros/json_no_automatic_udls.md) for the whole project and include
`<nlohmann/json_literals.hpp>` instead of `<nlohmann/json.hpp>` in the files that use the literals (it includes
`<nlohmann/json.hpp>` itself):
```cmake
target_compile_definitions(my_target PRIVATE JSON_NO_AUTOMATIC_UDLS)
```
```cpp
#include <nlohmann/json_literals.hpp> // only where "..."_json is used; includes <nlohmann/json.hpp>
```
The saving applies to translation units that do not parse JSON, for example ones that define types and their
conversions or only pass `json` values around:
| Compiler | Translation unit | Default (`-O0` / `-O2`) | `JSON_NO_AUTOMATIC_UDLS` (`-O0` / `-O2`) | Change |
|-------------|------------------|------------------------:|-----------------------------------------:|------------:|
| Apple clang | model | 776 ms / 846 ms | 629 ms / 692 ms | −19% / −18% |
| GCC 16 | model | 1022 ms / 1120 ms | 882 ms / 965 ms | −14% / −14% |
| Apple clang | parsing | 992 ms / 1815 ms | 1006 ms / 1823 ms | +1% / 0% |
| GCC 16 | parsing | 2018 ms / 3420 ms | 1990 ms / 3454 ms | −1% / +1% |
Translation units that include only the header save up to a third. Translation units that parse anyway instantiate
the parser regardless and see no difference.
## Instantiate `basic_json` once
Each translation unit instantiates the member functions of `nlohmann::json` it uses. An explicit instantiation
declaration tells the compiler that the non-template members are instantiated elsewhere, so it can skip them:
```cpp title="json_instance.hpp"
#pragma once
#include <nlohmann/json.hpp>
extern template class nlohmann::basic_json<>;
```
```cpp title="json_instance.cpp"
#include "json_instance.hpp"
template class nlohmann::basic_json<>;
```
Include `json_instance.hpp` instead of `<nlohmann/json.hpp>` and compile and link `json_instance.cpp` once.
| Compiler | Translation unit | Default (`-O0` / `-O2`) | `extern template` (`-O0` / `-O2`) | Change |
|-------------|---------------------|------------------------:|----------------------------------:|------------:|
| Apple clang | parsing | 992 ms / 1815 ms | 953 ms / 1625 ms | −4% / −10% |
| GCC 16 | parsing | 2018 ms / 3420 ms | 1522 ms / 2728 ms | −25% / −20% |
| Apple clang | `json_instance.cpp` | — | 2166 ms / 4660 ms | — |
| GCC 16 | `json_instance.cpp` | — | 5085 ms / 10616 ms | — |
Notes:
- The saving grows with the number of translation units that use `json`, while the instantiation translation unit is
compiled only once (and is rarely recompiled, as it does not depend on your code).
- Member function templates (such as `get<T>()`, `parse(InputType&&)`, or `value(key, default)`) are not covered by
the explicit instantiation and are still instantiated where they are used.
- The declaration covers exactly `nlohmann::json`. Add the same lines for `nlohmann::ordered_json`
(`nlohmann::basic_json<nlohmann::ordered_map>`) or your own `basic_json` specializations if you use them.
## 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](../features/modules.md) for requirements and known
issues. Module support is experimental and currently depends heavily on the compiler version.
## Use precompiled headers
Build systems can precompile `<nlohmann/json.hpp>` together with other stable headers, for example with CMake's
[`target_precompile_headers`](https://cmake.org/cmake/help/latest/command/target_precompile_headers.html):
```cmake
target_precompile_headers(my_target PRIVATE <nlohmann/json.hpp>)
```
This removes the cost of parsing the header, but not of instantiating templates in each translation unit, so it
combines well with the options above.
## Options without effect on compile times
Some configuration macros change what the library declares, but do not measurably change compile times:
| Macro | Apple clang, model (`-O0` / `-O2`) | GCC 16, model (`-O0` / `-O2`) |
|------------------------------------------------------------------------|-----------------------------------:|------------------------------:|
| default | 776 ms / 846 ms | 1022 ms / 1120 ms |
| [`JSON_NO_IO`](../api/macros/json_no_io.md) | 764 ms / 836 ms | 1022 ms / 1117 ms |
| [`JSON_USE_GLOBAL_UDLS`](../api/macros/json_use_global_udls.md)`=0` | 763 ms / 852 ms | 1019 ms / 1106 ms |
`JSON_USE_GLOBAL_UDLS` only controls *where* the literals are declared; to avoid their cost, use
`JSON_NO_AUTOMATIC_UDLS` instead.
## See also
- [`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
- [Header only](index.md) - including the library
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<!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=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-9.7.7"><title>Compile times - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> Line truncated
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json_fwd.hpp&gt;</span>
<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>&amp;</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>&amp;</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>&amp;</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>&amp;</span><span class=w> </span><span class=n>p</span><span class=p>);</span>
</code></pre></div> <div class=highlight><span class=filename>person.cpp</span><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>"person.hpp"</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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>&amp;</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>&amp;</span><span class=w> </span><span class=n>p</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w> </span><span class=cm>/* ... */</span><span class=w> </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>&amp;</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>&amp;</span><span class=w> </span><span class=n>p</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w> </span><span class=cm>/* ... */</span><span class=w> </span><span class=p>}</span>
</code></pre></div> <table> <thead> <tr> <th>Compiler</th> <th style="text-align: right;"><code>json.hpp</code> (<code>-O0</code>)</th> <th style="text-align: right;"><code>json_fwd.hpp</code> (<code>-O0</code>)</th> <th style="text-align: right;">Change</th> </tr> </thead> <tbody> <tr> <td>Apple clang</td> <td style="text-align: right;">704 ms</td> <td style="text-align: right;">329 ms</td> <td style="text-align: right;">−53%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td style="text-align: right;">779 ms</td> <td style="text-align: right;">242 ms</td> <td style="text-align: right;">−69%</td> </tr> </tbody> </table> <p>This is the most effective option, because it avoids the full header in every translation unit that includes <em>your</em> headers.</p> <h2 id=opt-out-of-the-automatic-user-defined-string-literals>Opt out of the automatic user-defined string literals<a class=headerlink href=#opt-out-of-the-automatic-user-defined-string-literals title="Permanent link">&para;</a></h2> <p>The user-defined string literals <a href=../../api/operator_literal_json/ ><code>operator""_json</code></a> and <a href=../../api/operator_literal_json_pointer/ ><code>operator""_json_pointer</code></a> are ordinary inline functions whose bodies call the parser. As <code>&lt;nlohmann/json.hpp&gt;</code> includes them by default, every translation unit instantiates the parser, even if it never parses anything itself.</p> <p>Define <a href=../../api/macros/json_no_automatic_udls/ ><code>JSON_NO_AUTOMATIC_UDLS</code></a> for the whole project and include <code>&lt;nlohmann/json_literals.hpp&gt;</code> instead of <code>&lt;nlohmann/json.hpp&gt;</code> in the files that use the literals (it includes <code>&lt;nlohmann/json.hpp&gt;</code> itself):</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_NO_AUTOMATIC_UDLS</span><span class=p>)</span>
</code></pre></div> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json_literals.hpp&gt;</span><span class=w> </span><span class=c1>// only where "..."_json is used; includes &lt;nlohmann/json.hpp&gt;</span>
</code></pre></div> <p>The saving applies to translation units that do not parse <abbr title="JavaScript Object Notation">JSON</abbr>, for example ones that define types and their conversions or only pass <code>json</code> values around:</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>JSON_NO_AUTOMATIC_UDLS</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;">776 ms / 846 ms</td> <td style="text-align: right;">629 ms / 692 ms</td> <td style="text-align: right;">−19% / −18%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>model</td> <td style="text-align: right;">1022 ms / 1120 ms</td> <td style="text-align: right;">882 ms / 965 ms</td> <td style="text-align: right;">−14% / −14%</td> </tr> <tr> <td>Apple clang</td> <td>parsing</td> <td style="text-align: right;">992 ms / 1815 ms</td> <td style="text-align: right;">1006 ms / 1823 ms</td> <td style="text-align: right;">+1% / 0%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>parsing</td> <td style="text-align: right;">2018 ms / 3420 ms</td> <td style="text-align: right;">1990 ms / 3454 ms</td> <td style="text-align: right;">−1% / +1%</td> </tr> </tbody> </table> <p>Translation units that include only the header save up to a third. Translation units that parse anyway instantiate the parser regardless and see no difference.</p> <h2 id=instantiate-basic_json-once>Instantiate <code>basic_json</code> once<a class=headerlink href=#instantiate-basic_json-once title="Permanent link">&para;</a></h2> <p>Each translation unit instantiates the member functions of <code>nlohmann::json</code> it uses. An explicit instantiation declaration tells the compiler that the non-template members are instantiated elsewhere, so it can skip them:</p> <div class=highlight><span class=filename>json_instance.hpp</span><pre><span></span><code><span class=cp>#pragma once</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
<span class=k>extern</span><span class=w> </span><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>&lt;&gt;</span><span class=p>;</span>
</code></pre></div> <div class=highlight><span class=filename>json_instance.cpp</span><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>"json_instance.hpp"</span>
<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>&lt;&gt;</span><span class=p>;</span>
</code></pre></div> <p>Include <code>json_instance.hpp</code> instead of <code>&lt;nlohmann/json.hpp&gt;</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;">992 ms / 1815 ms</td> <td style="text-align: right;">953 ms / 1625 ms</td> <td style="text-align: right;">−4% / −10%</td> </tr> <tr> <td><abbr title="GNU Compiler Collection">GCC</abbr> 16</td> <td>parsing</td> <td style="text-align: right;">2018 ms / 3420 ms</td> <td style="text-align: right;">1522 ms / 2728 ms</td> <td style="text-align: right;">−25% / −20%</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;">2166 ms / 4660 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;">5085 ms / 10616 ms</td> <td style="text-align: right;">—</td> </tr> </tbody> </table> <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&lt;T&gt;()</code>, <code>parse(InputType&amp;&amp;)</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&lt;nlohmann::ordered_map&gt;</code>) or your own <code>basic_json</code> specializations if you use them.</li> </ul> <h2 id=use-c20-modules>Use C++20 modules<a class=headerlink href=#use-c20-modules title="Permanent link">&para;</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> <h2 id=use-precompiled-headers>Use precompiled headers<a class=headerlink href=#use-precompiled-headers title="Permanent link">&para;</a></h2> <p>Build systems can precompile <code>&lt;nlohmann/json.hpp&gt;</code> together with other stable headers, for example with CMake's <a href="https://cmake.org/cmake/help/latest/command/target_precompile_headers.html"><code>target_precompile_headers</code></a>:</p> <div class=highlight><pre><span></span><code><span class=nb>target_precompile_headers</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>&lt;nlohmann/json.hpp&gt;</span><span class=p>)</span>
</code></pre></div> <p>This removes the cost of parsing the header, but not of instantiating templates in each translation unit, so it combines well with the options above.</p> <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">&para;</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;">776 ms / 846 ms</td> <td style="text-align: right;">1022 ms / 1120 ms</td> </tr> <tr> <td><a href=../../api/macros/json_no_io/ ><code>JSON_NO_IO</code></a></td> <td style="text-align: right;">764 ms / 836 ms</td> <td style="text-align: right;">1022 ms / 1117 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;">763 ms / 852 ms</td> <td style="text-align: right;">1019 ms / 1106 ms</td> </tr> </tbody> </table> <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">&para;</a></h2> <ul> <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> </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 4, 2026 10:23:53 UTC">October 4, 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> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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# Compile times
The library is header-only and makes heavy use of templates, so every translation unit that includes `<nlohmann/json.hpp>` pays for parsing the header and instantiating what it uses. This page lists the options to reduce that cost, ordered by how much they typically save.
Measurements
The numbers below are medians of nine runs compiling a single translation unit with `-std=c++17 -c` against the single-header version, with Apple clang and GCC 16 on macOS (Apple silicon). They show the order of magnitude to expect; measure your own code before and after a change.
## Include `json_fwd.hpp` in headers
Header files that only need to *name* the `json` type — for function declarations, members held by pointer or reference, or friend declarations — can include `<nlohmann/json_fwd.hpp>` instead of `<nlohmann/json.hpp>`. It only forward-declares `basic_json`, `json`, `ordered_json`, `json_pointer`, and `adl_serializer`. The translation units that actually use the values then include `<nlohmann/json.hpp>`.
person.hpp
```
#pragma once
#include <nlohmann/json_fwd.hpp>
struct person;
void to_json(nlohmann::json& j, const person& p);
void from_json(const nlohmann::json& j, person& p);
```
person.cpp
```
#include "person.hpp"
#include <nlohmann/json.hpp>
void to_json(nlohmann::json& j, const person& p) { /* ... */ }
void from_json(const nlohmann::json& j, person& p) { /* ... */ }
```
| Compiler | `json.hpp` (`-O0`) | `json_fwd.hpp` (`-O0`) | Change |
| ----------- | ------------------ | ---------------------- | ------ |
| Apple clang | 704 ms | 329 ms | −53% |
| GCC 16 | 779 ms | 242 ms | −69% |
This is the most effective option, because it avoids the full header in every translation unit that includes *your* headers.
## Opt out of the automatic user-defined string literals
The user-defined string literals [`operator""_json`](https://json.nlohmann.me/api/operator_literal_json/index.md) and [`operator""_json_pointer`](https://json.nlohmann.me/api/operator_literal_json_pointer/index.md) are ordinary inline functions whose bodies call the parser. As `<nlohmann/json.hpp>` includes them by default, every translation unit instantiates the parser, even if it never parses anything itself.
Define [`JSON_NO_AUTOMATIC_UDLS`](https://json.nlohmann.me/api/macros/json_no_automatic_udls/index.md) for the whole project and include `<nlohmann/json_literals.hpp>` instead of `<nlohmann/json.hpp>` in the files that use the literals (it includes `<nlohmann/json.hpp>` itself):
```
target_compile_definitions(my_target PRIVATE JSON_NO_AUTOMATIC_UDLS)
```
```
#include <nlohmann/json_literals.hpp> // only where "..."_json is used; includes <nlohmann/json.hpp>
```
The saving applies to translation units that do not parse JSON, for example ones that define types and their conversions or only pass `json` values around:
| Compiler | Translation unit | Default (`-O0` / `-O2`) | `JSON_NO_AUTOMATIC_UDLS` (`-O0` / `-O2`) | Change |
| ----------- | ---------------- | ----------------------- | ---------------------------------------- | ----------- |
| Apple clang | model | 776 ms / 846 ms | 629 ms / 692 ms | −19% / −18% |
| GCC 16 | model | 1022 ms / 1120 ms | 882 ms / 965 ms | −14% / −14% |
| Apple clang | parsing | 992 ms / 1815 ms | 1006 ms / 1823 ms | +1% / 0% |
| GCC 16 | parsing | 2018 ms / 3420 ms | 1990 ms / 3454 ms | −1% / +1% |
Translation units that include only the header save up to a third. Translation units that parse anyway instantiate the parser regardless and see no difference.
## Instantiate `basic_json` once
Each translation unit instantiates the member functions of `nlohmann::json` it uses. An explicit instantiation declaration tells the compiler that the non-template members are instantiated elsewhere, so it can skip them:
json_instance.hpp
```
#pragma once
#include <nlohmann/json.hpp>
extern template class nlohmann::basic_json<>;
```
json_instance.cpp
```
#include "json_instance.hpp"
template class nlohmann::basic_json<>;
```
Include `json_instance.hpp` instead of `<nlohmann/json.hpp>` and compile and link `json_instance.cpp` once.
| Compiler | Translation unit | Default (`-O0` / `-O2`) | `extern template` (`-O0` / `-O2`) | Change |
| ----------- | ------------------- | ----------------------- | --------------------------------- | ----------- |
| Apple clang | parsing | 992 ms / 1815 ms | 953 ms / 1625 ms | −4% / −10% |
| GCC 16 | parsing | 2018 ms / 3420 ms | 1522 ms / 2728 ms | −25% / −20% |
| Apple clang | `json_instance.cpp` | — | 2166 ms / 4660 ms | — |
| GCC 16 | `json_instance.cpp` | — | 5085 ms / 10616 ms | — |
Notes:
- The saving grows with the number of translation units that use `json`, while the instantiation translation unit is compiled only once (and is rarely recompiled, as it does not depend on your code).
- Member function templates (such as `get<T>()`, `parse(InputType&&)`, or `value(key, default)`) are not covered by the explicit instantiation and are still instantiated where they are used.
- The declaration covers exactly `nlohmann::json`. Add the same lines for `nlohmann::ordered_json` (`nlohmann::basic_json<nlohmann::ordered_map>`) or your own `basic_json` specializations if you use them.
## 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.
## Use precompiled headers
Build systems can precompile `<nlohmann/json.hpp>` together with other stable headers, for example with CMake's [`target_precompile_headers`](https://cmake.org/cmake/help/latest/command/target_precompile_headers.html):
```
target_precompile_headers(my_target PRIVATE <nlohmann/json.hpp>)
```
This removes the cost of parsing the header, but not of instantiating templates in each translation unit, so it combines well with the options above.
## Options without effect on compile times
Some configuration macros change what the library declares, but do not measurably change compile times:
| Macro | Apple clang, model (`-O0` / `-O2`) | GCC 16, model (`-O0` / `-O2`) |
| ----------------------------------------------------------------------------------------------- | ---------------------------------- | ----------------------------- |
| default | 776 ms / 846 ms | 1022 ms / 1120 ms |
| [`JSON_NO_IO`](https://json.nlohmann.me/api/macros/json_no_io/index.md) | 764 ms / 836 ms | 1022 ms / 1117 ms |
| [`JSON_USE_GLOBAL_UDLS`](https://json.nlohmann.me/api/macros/json_use_global_udls/index.md)`=0` | 763 ms / 852 ms | 1019 ms / 1106 ms |
`JSON_USE_GLOBAL_UDLS` only controls *where* the literals are declared; to avoid their cost, use `JSON_NO_AUTOMATIC_UDLS` instead.
## See also
- [`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
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<!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=author content="Niels Lohmann"><link href="https://json.nlohmann.me/integration/" rel="canonical"><link href=../features/types/template_parameters/ rel=prev><link href=migration_guide/ rel=next><link rel=icon href=../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Integration - JSON for Modern C++</title><link rel=stylesheet href=../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../css/custom.css><script>__md_scope=new 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A[Add the library to a C++ project] --&gt; B{Already using CMake?}
B -- no --&gt; C{Using pkg-config or plain Makefiles?}
C -- yes --&gt; D[pkg-config]
@@ -11,4 +11,4 @@
<span class=c1>// for convenience</span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
</code></pre></div> <p>to the files you want to process <abbr title="JavaScript Object Notation">JSON</abbr> and set the necessary switches to enable C++11 (e.g., <code>-std=c++11</code> for <abbr title="GNU Compiler Collection">GCC</abbr> and Clang).</p> <p>You can further use file <a href="https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json_fwd.hpp"><code>single_include/nlohmann/json_fwd.hpp</code></a> for forward declarations, and file <a href="https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json_literals.hpp"><code>single_include/nlohmann/json_literals.hpp</code></a> for the user-defined string literals if you define <a href=../api/macros/json_no_automatic_udls/ ><code>JSON_NO_AUTOMATIC_UDLS</code></a>.</p> <!-- 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 2, 2026 09:32:15 UTC">October 2, 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> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> <p>to the files you want to process <abbr title="JavaScript Object Notation">JSON</abbr> and set the necessary switches to enable C++11 (e.g., <code>-std=c++11</code> for <abbr title="GNU Compiler Collection">GCC</abbr> and Clang).</p> <p>You can further use file <a href="https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json_fwd.hpp"><code>single_include/nlohmann/json_fwd.hpp</code></a> for forward declarations (see <a href=compile_times/ >Compile times</a>), and file <a href="https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json_literals.hpp"><code>single_include/nlohmann/json_literals.hpp</code></a> for the user-defined string literals if you define <a href=../api/macros/json_no_automatic_udls/ ><code>JSON_NO_AUTOMATIC_UDLS</code></a>.</p> <!-- 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 4, 2026 10:23:53 UTC">October 4, 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> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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+1 -1
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@@ -35,4 +35,4 @@ using json = nlohmann::json;
to the files you want to process JSON and set the necessary switches to enable C++11 (e.g., `-std=c++11` for GCC and Clang).
You can further use file [`single_include/nlohmann/json_fwd.hpp`](https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json_fwd.hpp) for forward declarations, and file [`single_include/nlohmann/json_literals.hpp`](https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json_literals.hpp) for the user-defined string literals if you define [`JSON_NO_AUTOMATIC_UDLS`](https://json.nlohmann.me/api/macros/json_no_automatic_udls/index.md).
You can further use file [`single_include/nlohmann/json_fwd.hpp`](https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json_fwd.hpp) for forward declarations (see [Compile times](https://json.nlohmann.me/integration/compile_times/index.md)), and file [`single_include/nlohmann/json_literals.hpp`](https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json_literals.hpp) for the user-defined string literals if you define [`JSON_NO_AUTOMATIC_UDLS`](https://json.nlohmann.me/api/macros/json_no_automatic_udls/index.md).
+5 -2
View File
@@ -2,11 +2,14 @@
This page collects some guidelines on how to future-proof your code for future versions of this library. For how to
add the library to your project in the first place, see [Integration](index.md), [CMake](cmake.md), or
[Package Managers](package_managers.md).
[Package Managers](package_managers.md). The [roadmap](../community/roadmap.md#version-40) lists what will change in
version 4.0, including the macros that let you try its behavior with a 3.x release; this page describes how to adjust
your code.
## Replace deprecated functions
The following functions have been deprecated and will be removed in the next major version (i.e., 4.0.0). All
The following functions have been deprecated and will be removed in the next major version (i.e., 4.0.0), see the
[roadmap](../community/roadmap.md#removal-of-deprecated-functions) for an overview. All
deprecations are annotated with
[`HEDLEY_DEPRECATED_FOR`](https://nemequ.github.io/hedley/api-reference.html#HEDLEY_DEPRECATED_FOR) to report which
function to use instead.
+2 -2
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@@ -1,4 +1,4 @@
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<span class=n>std</span><span class=o>::</span><span class=n>stringstream</span><span class=w> </span><span class=n>ss</span><span class=p>(</span><span class=s>"[1,2,3]"</span><span class=p>);</span>
<span class=n>j</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>ss</span><span class=p>;</span>
</code></pre></div> </div> <div class=tabbed-block> <div class=highlight><pre><span></span><code><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span>
@@ -54,4 +54,4 @@
<span class=p>{</span>
<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"age"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>p</span><span class=p>.</span><span class=n>age</span><span class=p>;</span>
<span class=p>}</span>
</code></pre></div> </div> </div> </div> <h2 id=do-not-use-the-detail-namespace>Do not use the <code>detail</code> namespace<a class=headerlink href=#do-not-use-the-detail-namespace title="Permanent link">&para;</a></h2> <p>The <code>nlohmann::detail</code> namespace is not part of the public <abbr title="Application Programming Interfaces">API</abbr> of the library and can change in any version without an announcement. 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</code></pre></div> </div> </div> </div> <h2 id=do-not-use-the-detail-namespace>Do not use the <code>detail</code> namespace<a class=headerlink href=#do-not-use-the-detail-namespace title="Permanent link">&para;</a></h2> <p>The <code>nlohmann::detail</code> namespace is not part of the public <abbr title="Application Programming Interfaces">API</abbr> of the library and can change in any version without an announcement. Do not rely on any function or type in the <code>detail</code> namespace.</p> <!-- 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 4, 2026 09:59:34 UTC">October 4, 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> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1m-11.4-14.7c-1.6 1-1.6 3.6 0 5.9s4.3 3.3 5.6 2.3c1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2"/></svg> </a> <a href="https://www.linkedin.com/in/nielslohmann/" target="_blank" rel="noopener" title="www.linkedin.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 448 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M416 32H31.9C14.3 32 0 46.5 0 64.3v383.4C0 465.5 14.3 480 31.9 480H416c17.6 0 32-14.5 32-32.3V64.3c0-17.8-14.4-32.3-32-32.3M135.4 416H69V202.2h66.5V416zM102.2 96a38.5 38.5 0 1 1 0 77 38.5 38.5 0 1 1 0-77m282.1 320h-66.4V312c0-24.8-.5-56.7-34.5-56.7-34.6 0-39.9 27-39.9 54.9V416h-66.4V202.2h63.7v29.2h.9c8.9-16.8 30.6-34.5 62.9-34.5 67.2 0 79.7 44.3 79.7 101.9z"/></svg> </a> <a href="https://www.xing.com/profile/Niels_Lohmann" target="_blank" rel="noopener" title="www.xing.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 384 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - Line truncated
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# Migration Guide
This page collects some guidelines on how to future-proof your code for future versions of this library. For how to add the library to your project in the first place, see [Integration](https://json.nlohmann.me/integration/index.md), [CMake](https://json.nlohmann.me/integration/cmake/index.md), or [Package Managers](https://json.nlohmann.me/integration/package_managers/index.md).
This page collects some guidelines on how to future-proof your code for future versions of this library. For how to add the library to your project in the first place, see [Integration](https://json.nlohmann.me/integration/index.md), [CMake](https://json.nlohmann.me/integration/cmake/index.md), or [Package Managers](https://json.nlohmann.me/integration/package_managers/index.md). The [roadmap](https://json.nlohmann.me/community/roadmap/#version-40) lists what will change in version 4.0, including the macros that let you try its behavior with a 3.x release; this page describes how to adjust your code.
## Replace deprecated functions
The following functions have been deprecated and will be removed in the next major version (i.e., 4.0.0). All deprecations are annotated with [`HEDLEY_DEPRECATED_FOR`](https://nemequ.github.io/hedley/api-reference.html#HEDLEY_DEPRECATED_FOR) to report which function to use instead.
The following functions have been deprecated and will be removed in the next major version (i.e., 4.0.0), see the [roadmap](https://json.nlohmann.me/community/roadmap/#removal-of-deprecated-functions) for an overview. All deprecations are annotated with [`HEDLEY_DEPRECATED_FOR`](https://nemequ.github.io/hedley/api-reference.html#HEDLEY_DEPRECATED_FOR) to report which function to use instead.
### Parsing
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<!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=author content="Niels Lohmann"><link href="https://json.nlohmann.me/integration/package_managers/" rel="canonical"><link href=../cmake/ rel=prev><link href=../pkg-config/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Package Managers - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
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</code></pre></div> <p>A pkg-config file is installed by <a href=../cmake/#json_install>CMake</a> (when the <code>JSON_Install</code> option is enabled, which is the default for a top-level build) as well as by several <a href=../package_managers/ >package managers</a>.</p> <p>Users of the <a href=../package_managers/#meson>Meson build system</a> will also be able to use a system-wide library, which will be found by <code>pkg-config</code>:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=nb>dependency</span><span class=p>(</span><span class=s>'nlohmann_json'</span><span class=p>,</span><span class=w> </span><span class=n>required</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>)</span>
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