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@@ -305,6 +305,51 @@ Only very old NDKs (before r18), which defaulted to GCC and `gnustl`, lacked C++
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`std::to_string`. If you run into this, update to a current NDK.
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### Incomplete `detector` type with GCC < 11
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!!! question
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Why does GCC 10 or older fail with `invalid use of incomplete type 'struct nlohmann::detail::detector<..., to_json_function, ...>'` for a type that holds an `optional` member?
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This happens with GCC 10 and older in C++11/C++14 mode when all of these hold:
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- a class `Holder` has an `optional<Dummy>` member (e.g., `boost::optional`),
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- `Dummy` has a constructor taking a `json` value, and
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- `to_json` for `Holder` is a free function in the namespace of `Dummy`.
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```cpp
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class Dummy {
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public:
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explicit Dummy(const nlohmann::json& j);
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};
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class Holder {
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boost::optional<Dummy> d;
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};
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void to_json(nlohmann::json& j, const Holder& h); // triggers the error
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```
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To decide whether `Dummy` is copyable, the compiler checks whether a `Dummy` can be converted to `json`. That check
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looks up `to_json` via argument-dependent lookup, finds the unrelated `to_json` for `Holder`, and eventually asks again
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whether `Dummy` is copyable. GCC before version 11 turns this cycle into a hard error; GCC 11 and later, Clang, and
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C++17 mode compile the code. The same error shows up without this library whenever a constrained converting constructor
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is involved, so the library can't avoid it.
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To work around this, define `to_json` (and `from_json`) as a *hidden friend* inside the class. That way,
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argument-dependent lookup only finds it for `Holder`:
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```cpp
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class Holder {
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boost::optional<Dummy> d;
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friend void to_json(nlohmann::json& j, const Holder& h) { /* ... */ }
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};
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```
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The [`NLOHMANN_DEFINE_TYPE_INTRUSIVE`](../api/macros/nlohmann_define_type_intrusive.md) macros define hidden friends as
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well. See [#3669](https://github.com/nlohmann/json/issues/3669) for details.
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### Missing STL function
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!!! question "Questions"
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+17
-2
@@ -1,4 +1,4 @@
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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/home/faq/" rel="canonical"><link href=../license/ rel=prev><link href=../exceptions/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>FAQ - 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=#frequently-asked-questions-faq 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> FAQ </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" 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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-md-cLine truncated
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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/home/faq/" rel="canonical"><link href=../license/ rel=prev><link href=../exceptions/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>FAQ - 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=#frequently-asked-questions-faq 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> FAQ </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" 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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-md-cLine truncated
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</code></pre></div> <p>and</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=nf>j</span><span class=p>(</span><span class=nb>true</span><span class=p>);</span>
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</code></pre></div> <p>yield different results (<code class=highlight><span class=p>[</span><span class=kc>true</span><span class=p>]</span></code> vs. <code class=highlight><span class=kc>true</span></code>)?</p> </div> <p>This is a known issue, and -- even worse -- the behavior differs between <abbr title="GNU Compiler Collection">GCC</abbr> and Clang. The "culprit" for this is the library's constructor overloads for initializer lists to allow syntax like</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>array</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=mi>3</span><span class=p>,</span><span class=w> </span><span class=mi>4</span><span class=p>};</span>
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</code></pre></div> <p>for arrays and</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>object</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{{</span><span class=s>"one"</span><span class=p>,</span><span class=w> </span><span class=mi>1</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"two"</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>}};</span><span class=w> </span>
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@@ -41,4 +41,19 @@
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<span class=p>}</span>
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</code></pre></div> </div> <h2 id=usage>Usage<a class=headerlink href=#usage title="Permanent link">¶</a></h2> <h3 id=thread-safety>Thread safety<a class=headerlink href=#thread-safety title="Permanent link">¶</a></h3> <div class="admonition question"> <p class=admonition-title>Question</p> <p>Is <code>basic_json</code> thread-safe?</p> </div> <p>No. <code>basic_json</code> provides no built-in synchronization, the same as <code>std::map</code> or <code>std::vector</code>. Concurrent reads of the same value from multiple threads are safe, as are concurrent (non-overlapping) accesses to independent <code>json</code> objects. However, any concurrent write to a <code>json</code> object -- or a concurrent read while another thread writes to the same object -- is a data race and requires external synchronization (e.g., a <code>std::mutex</code>) by the caller.</p> <h3 id=schema-validation>Schema validation<a class=headerlink href=#schema-validation title="Permanent link">¶</a></h3> <div class="admonition question"> <p class=admonition-title>Question</p> <p>Does this library support <abbr title="JavaScript Object Notation">JSON</abbr> Schema validation?</p> </div> <p>Not directly, but the companion project <a href="https://github.com/pboettch/json-schema-validator">json-schema-validator</a> builds <abbr title="JavaScript Object Notation">JSON</abbr> Schema (draft 7; draft 4 in its older, now-superseded 1.x releases) validation on top of this library and is a common recommendation for this use case.</p> <h2 id=exceptions>Exceptions<a class=headerlink href=#exceptions title="Permanent link">¶</a></h2> <h3 id=parsing-without-exceptions>Parsing without exceptions<a class=headerlink href=#parsing-without-exceptions title="Permanent link">¶</a></h3> <div class="admonition question"> <p class=admonition-title>Question</p> <p>Is it possible to indicate a parse error without throwing an exception?</p> </div> <p>Yes, see <a href=../../features/parsing/parse_exceptions/ >Parsing and exceptions</a>.</p> <h3 id=key-name-in-exceptions>Key name in exceptions<a class=headerlink href=#key-name-in-exceptions title="Permanent link">¶</a></h3> <div class="admonition question"> <p class=admonition-title>Question</p> <p>Can I get the key of the object item that caused an exception?</p> </div> <p>Yes, you can. Please define the symbol <a href=../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a> to get <a href=../exceptions/#extended-diagnostic-messages>extended diagnostics messages</a>.</p> <h2 id=serialization-issues>Serialization issues<a class=headerlink href=#serialization-issues title="Permanent link">¶</a></h2> <h3 id=number-precision>Number precision<a class=headerlink href=#number-precision title="Permanent link">¶</a></h3> <div class="admonition question"> <p class=admonition-title>Question</p> <ul> <li>It seems that precision is lost when serializing a double.</li> <li>Can I change the precision for floating-point serialization?</li> </ul> </div> <p>The library uses <code>std::numeric_limits<number_float_t>::digits10</code> (15 for <abbr title="Institute of Electrical and Electronics Engineers">IEEE</abbr> <code>double</code>s) digits for serialization. This value is sufficient to guarantee roundtripping. If one uses more than this number of digits of precision, then string -> value -> string is not guaranteed to round-trip.</p> <div class="admonition quote"> <p class=admonition-title><a href="https://en.cppreference.com/w/cpp/types/numeric_limits/digits10">cppreference.com</a></p> <p>The value of <code>std::numeric_limits<T>::digits10</code> is the number of base-10 digits that can be represented by the type T without change, that is, any number with this many significant decimal digits can be converted to a value of type T and back to decimal form, without change due to rounding or overflow. </p> </div> <div class="admonition tip"> <p class=admonition-title>Tip</p> <p>The website <a href="https://float.exposed">https://float.exposed</a> gives a good insight into the internal storage of floating-point numbers.</p> </div> <p>See <a href=../../features/types/number_handling/#number-serialization>this section</a> on the library's number handling for more information.</p> <h3 id=serializing-untrusted-or-invalid-utf-8>Serializing untrusted or invalid <abbr title="Unicode Transformation Format">UTF</abbr>-8<a class=headerlink href=#serializing-untrusted-or-invalid-utf-8 title="Permanent link">¶</a></h3> <div class="admonition question"> <p class=admonition-title>Questions</p> <ul> <li>Why does <code>dump()</code> throw when I serialize data that came from the network?</li> <li>Is CVE-2024-34363 a vulnerability in this library?</li> </ul> </div> <p>Crashes reported against this library that stem from an uncaught <a href=../exceptions/#jsonexceptiontype_error316><code>type_error.316</code></a> while serializing unvalidated input (e.g., CVE-2024-34363) aLine truncated
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||||
<span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>s</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>,</span><span class=w> </span><span class=sc>' '</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>error_handler_t</span><span class=o>::</span><span class=n>replace</span><span class=p>);</span>
|
||||
</code></pre></div> <h3 id=using-json-values-with-stdformat-or-fmt>Using <abbr title="JavaScript Object Notation">JSON</abbr> values with <code>std::format</code> or <code>fmt</code><a class=headerlink href=#using-json-values-with-stdformat-or-fmt title="Permanent link">¶</a></h3> <div class="admonition question"> <p class=admonition-title>Question</p> <ul> <li>Can I use <code>std::format("{}", j)</code> on a <abbr title="JavaScript Object Notation">JSON</abbr> value?</li> <li>Can I use <code>fmt::format("{}", j)</code> or <code>fmt::print("{}", j)</code> (the <a href="https://github.com/fmtlib/fmt">{fmt}</a> library) on a <abbr title="JavaScript Object Notation">JSON</abbr> value?</li> </ul> </div> <p><code>std::format</code> works out of the box since version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>, as long as the standard library provides <code><format></code> (see <a href=../../api/macros/json_has_std_format/ ><code>JSON_HAS_STD_FORMAT</code></a>); see <a href=../../api/basic_json/std_formatter/ ><code>std::formatter<basic_json></code></a> for details, including the <code class=highlight><span class=s>"{:#}"</span></code> pretty-print spec, indent widths (<code class=highlight><span class=s>"{:2}"</span></code>), and custom indent characters (<code class=highlight><span class=s>"{:.>#}"</span></code>).</p> <p>For <code>fmt</code>, the library ships <a href=../../api/basic_json/format_as/ ><code>format_as</code></a>, a small customization point <code>fmt</code> looks for via argument-dependent lookup. It only has an effect on fmt 10.0.0 through 11.0.2 — from fmt 11.1.0 onwards, <code>fmt</code> no longer picks up a <code>format_as</code> overload that returns a <code>std::string</code>. On such versions (or any version, if you also want the same <code class=highlight><span class=s>"{:#}"</span></code>/width/fill-and-align spec support that <code>std::formatter<basic_json></code> has), define your own <code>fmt::formatter</code> specialization; see <a href=../../api/basic_json/format_as/ ><code>format_as</code></a> for a recipe that mirrors it.</p> <p>If you get ambiguous-overload errors when passing a <abbr title="JavaScript Object Notation">JSON</abbr> value to <code>fmt::format</code>/<code>fmt::print</code> without any <code>fmt::formatter<json></code> specialization in scope, that's <code>fmt</code> picking up <code>basic_json</code>'s implicit <code>operator ValueType()</code> conversion operator (see <a href="https://github.com/nlohmann/json/issues/964">#964</a> and <a href="https://github.com/nlohmann/json/issues/958">#958</a>); disabling it via <a href=../../api/macros/json_use_implicit_conversions/ ><code>JSON_USE_IMPLICIT_CONVERSIONS 0</code></a> avoids the ambiguity.</p> <h2 id=compilation-issues>Compilation issues<a class=headerlink href=#compilation-issues title="Permanent link">¶</a></h2> <h3 id=android-sdk>Android <abbr title="Software Development Kit">SDK</abbr><a class=headerlink href=#android-sdk title="Permanent link">¶</a></h3> <div class="admonition question"> <p class=admonition-title>Question</p> <p>Why does the code not compile with Android <abbr title="Software Development Kit">SDK</abbr>?</p> </div> <p>Since <a href="https://github.com/android/ndk/wiki/Changelog-r18"><abbr title="Native Development Kit">NDK</abbr> r18</a> (2018), <abbr title="GNU Compiler Collection">GCC</abbr> and the <code>gnustl</code>/<code>stlport</code> C++ libraries have been removed from the Android <abbr title="Native Development Kit">NDK</abbr>; Clang and <code>libc++</code> are now the only compiler and C++ library, and they support C++11 and later out of the box. With a current <abbr title="Native Development Kit">NDK</abbr>, no special configuration is needed to use this library.</p> <p>Only very old NDKs (before r18), which defaulted to <abbr title="GNU Compiler Collection">GCC</abbr> and <code>gnustl</code>, lacked C++11 library features such as <code>std::to_string</code>. If you run into this, update to a current <abbr title="Native Development Kit">NDK</abbr>.</p> <h3 id=missing-stl-function>Missing <abbr title="Standard Template Library">STL</abbr> function<a class=headerlink href=#missing-stl-function title="Permanent link">¶</a></h3> <div class="admonition question"> <p class=admonition-title>Questions</p> <ul> <li>Why do I get a compilation error <code>'to_string' is not a member of 'std'</code> (or similarly, for <code>strtod</code> or <code>strtof</code>)?</li> <li>Why does the code not compile with MinGW or Android <abbr title="Software Development Kit">SDK</abbr>?</li> </ul> </div> <p>This is not an issue with the code, but rather with the compiler itself. On Android, use a current <abbr title="Native Development Kit">NDK</abbr> (see above). For MinGW, please refer to <a href="http://tehsausage.com/mingw-to-string">this site</a> and <a href="https://github.cLine truncated
|
||||
</code></pre></div> <h3 id=using-json-values-with-stdformat-or-fmt>Using <abbr title="JavaScript Object Notation">JSON</abbr> values with <code>std::format</code> or <code>fmt</code><a class=headerlink href=#using-json-values-with-stdformat-or-fmt title="Permanent link">¶</a></h3> <div class="admonition question"> <p class=admonition-title>Question</p> <ul> <li>Can I use <code>std::format("{}", j)</code> on a <abbr title="JavaScript Object Notation">JSON</abbr> value?</li> <li>Can I use <code>fmt::format("{}", j)</code> or <code>fmt::print("{}", j)</code> (the <a href="https://github.com/fmtlib/fmt">{fmt}</a> library) on a <abbr title="JavaScript Object Notation">JSON</abbr> value?</li> </ul> </div> <p><code>std::format</code> works out of the box since version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>, as long as the standard library provides <code><format></code> (see <a href=../../api/macros/json_has_std_format/ ><code>JSON_HAS_STD_FORMAT</code></a>); see <a href=../../api/basic_json/std_formatter/ ><code>std::formatter<basic_json></code></a> for details, including the <code class=highlight><span class=s>"{:#}"</span></code> pretty-print spec, indent widths (<code class=highlight><span class=s>"{:2}"</span></code>), and custom indent characters (<code class=highlight><span class=s>"{:.>#}"</span></code>).</p> <p>For <code>fmt</code>, the library ships <a href=../../api/basic_json/format_as/ ><code>format_as</code></a>, a small customization point <code>fmt</code> looks for via argument-dependent lookup. It only has an effect on fmt 10.0.0 through 11.0.2 — from fmt 11.1.0 onwards, <code>fmt</code> no longer picks up a <code>format_as</code> overload that returns a <code>std::string</code>. On such versions (or any version, if you also want the same <code class=highlight><span class=s>"{:#}"</span></code>/width/fill-and-align spec support that <code>std::formatter<basic_json></code> has), define your own <code>fmt::formatter</code> specialization; see <a href=../../api/basic_json/format_as/ ><code>format_as</code></a> for a recipe that mirrors it.</p> <p>If you get ambiguous-overload errors when passing a <abbr title="JavaScript Object Notation">JSON</abbr> value to <code>fmt::format</code>/<code>fmt::print</code> without any <code>fmt::formatter<json></code> specialization in scope, that's <code>fmt</code> picking up <code>basic_json</code>'s implicit <code>operator ValueType()</code> conversion operator (see <a href="https://github.com/nlohmann/json/issues/964">#964</a> and <a href="https://github.com/nlohmann/json/issues/958">#958</a>); disabling it via <a href=../../api/macros/json_use_implicit_conversions/ ><code>JSON_USE_IMPLICIT_CONVERSIONS 0</code></a> avoids the ambiguity.</p> <h2 id=compilation-issues>Compilation issues<a class=headerlink href=#compilation-issues title="Permanent link">¶</a></h2> <h3 id=android-sdk>Android <abbr title="Software Development Kit">SDK</abbr><a class=headerlink href=#android-sdk title="Permanent link">¶</a></h3> <div class="admonition question"> <p class=admonition-title>Question</p> <p>Why does the code not compile with Android <abbr title="Software Development Kit">SDK</abbr>?</p> </div> <p>Since <a href="https://github.com/android/ndk/wiki/Changelog-r18"><abbr title="Native Development Kit">NDK</abbr> r18</a> (2018), <abbr title="GNU Compiler Collection">GCC</abbr> and the <code>gnustl</code>/<code>stlport</code> C++ libraries have been removed from the Android <abbr title="Native Development Kit">NDK</abbr>; Clang and <code>libc++</code> are now the only compiler and C++ library, and they support C++11 and later out of the box. With a current <abbr title="Native Development Kit">NDK</abbr>, no special configuration is needed to use this library.</p> <p>Only very old NDKs (before r18), which defaulted to <abbr title="GNU Compiler Collection">GCC</abbr> and <code>gnustl</code>, lacked C++11 library features such as <code>std::to_string</code>. If you run into this, update to a current <abbr title="Native Development Kit">NDK</abbr>.</p> <h3 id=incomplete-detector-type-with-gcc-11>Incomplete <code>detector</code> type with <abbr title="GNU Compiler Collection">GCC</abbr> < 11<a class=headerlink href=#incomplete-detector-type-with-gcc-11 title="Permanent link">¶</a></h3> <div class="admonition question"> <p class=admonition-title>Question</p> <p>Why does <abbr title="GNU Compiler Collection">GCC</abbr> 10 or older fail with <code>invalid use of incomplete type 'struct nlohmann::detail::detector<..., to_json_function, ...>'</code> for a type that holds an <code>optional</code> member?</p> </div> <p>This happens with <abbr title="GNU Compiler Collection">GCC</abbr> 10 and older in C++11/C++14 mode when all of these hold:</p> <ul> <li>a class <code>Holder</code> has an <code>optional<Dummy></code> member (e.g., <code>boost::optiLine truncated
|
||||
<span class=w> </span><span class=k>public</span><span class=o>:</span>
|
||||
<span class=w> </span><span class=k>explicit</span><span class=w> </span><span class=n>Dummy</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=p>};</span>
|
||||
|
||||
<span class=k>class</span><span class=w> </span><span class=nc>Holder</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=n>boost</span><span class=o>::</span><span class=n>optional</span><span class=o><</span><span class=n>Dummy</span><span class=o>></span><span class=w> </span><span class=n>d</span><span class=p>;</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>Holder</span><span class=o>&</span><span class=w> </span><span class=n>h</span><span class=p>);</span><span class=w> </span><span class=c1>// triggers the error</span>
|
||||
</code></pre></div> <p>To decide whether <code>Dummy</code> is copyable, the compiler checks whether a <code>Dummy</code> can be converted to <code>json</code>. That check looks up <code>to_json</code> via argument-dependent lookup, finds the unrelated <code>to_json</code> for <code>Holder</code>, and eventually asks again whether <code>Dummy</code> is copyable. <abbr title="GNU Compiler Collection">GCC</abbr> before version 11 turns this cycle into a hard error; <abbr title="GNU Compiler Collection">GCC</abbr> 11 and later, Clang, and C++17 mode compile the code. The same error shows up without this library whenever a constrained converting constructor is involved, so the library can't avoid it.</p> <p>To work around this, define <code>to_json</code> (and <code>from_json</code>) as a <em>hidden friend</em> inside the class. That way, argument-dependent lookup only finds it for <code>Holder</code>:</p> <div class=highlight><pre><span></span><code><span class=k>class</span><span class=w> </span><span class=nc>Holder</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=n>boost</span><span class=o>::</span><span class=n>optional</span><span class=o><</span><span class=n>Dummy</span><span class=o>></span><span class=w> </span><span class=n>d</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=k>friend</span><span class=w> </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>Holder</span><span class=o>&</span><span class=w> </span><span class=n>h</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=p>};</span>
|
||||
</code></pre></div> <p>The <a href=../../api/macros/nlohmann_define_type_intrusive/ ><code>NLOHMANN_DEFINE_TYPE_INTRUSIVE</code></a> macros define hidden friends as well. See <a href="https://github.com/nlohmann/json/issues/3669">#3669</a> for details.</p> <h3 id=missing-stl-function>Missing <abbr title="Standard Template Library">STL</abbr> function<a class=headerlink href=#missing-stl-function title="Permanent link">¶</a></h3> <div class="admonition question"> <p class=admonition-title>Questions</p> <ul> <li>Why do I get a compilation error <code>'to_string' is not a member of 'std'</code> (or similarly, for <code>strtod</code> or <code>strtof</code>)?</li> <li>Why does the code not compile with MinGW or Android <abbr title="Software Development Kit">SDK</abbr>?</li> </ul> </div> <p>This is not an issue with the code, but rather with the compiler itself. On Android, use a current <abbr title="Native Development Kit">NDK</abbr> (see above). For MinGW, please refer to <a href="http://tehsausage.com/mingw-to-string">this site</a> and <a href="https://github.com/nlohmann/json/issues/136">this discussion</a> for information on how to fix this bug.</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 09:46:13 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 © 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, CodLine truncated
|
||||
@@ -273,6 +273,45 @@ Since [NDK r18](https://github.com/android/ndk/wiki/Changelog-r18) (2018), GCC a
|
||||
|
||||
Only very old NDKs (before r18), which defaulted to GCC and `gnustl`, lacked C++11 library features such as `std::to_string`. If you run into this, update to a current NDK.
|
||||
|
||||
### Incomplete `detector` type with GCC < 11
|
||||
|
||||
Question
|
||||
|
||||
Why does GCC 10 or older fail with `invalid use of incomplete type 'struct nlohmann::detail::detector<..., to_json_function, ...>'` for a type that holds an `optional` member?
|
||||
|
||||
This happens with GCC 10 and older in C++11/C++14 mode when all of these hold:
|
||||
|
||||
- a class `Holder` has an `optional<Dummy>` member (e.g., `boost::optional`),
|
||||
- `Dummy` has a constructor taking a `json` value, and
|
||||
- `to_json` for `Holder` is a free function in the namespace of `Dummy`.
|
||||
|
||||
```
|
||||
class Dummy {
|
||||
public:
|
||||
explicit Dummy(const nlohmann::json& j);
|
||||
};
|
||||
|
||||
class Holder {
|
||||
boost::optional<Dummy> d;
|
||||
};
|
||||
|
||||
void to_json(nlohmann::json& j, const Holder& h); // triggers the error
|
||||
```
|
||||
|
||||
To decide whether `Dummy` is copyable, the compiler checks whether a `Dummy` can be converted to `json`. That check looks up `to_json` via argument-dependent lookup, finds the unrelated `to_json` for `Holder`, and eventually asks again whether `Dummy` is copyable. GCC before version 11 turns this cycle into a hard error; GCC 11 and later, Clang, and C++17 mode compile the code. The same error shows up without this library whenever a constrained converting constructor is involved, so the library can't avoid it.
|
||||
|
||||
To work around this, define `to_json` (and `from_json`) as a *hidden friend* inside the class. That way, argument-dependent lookup only finds it for `Holder`:
|
||||
|
||||
```
|
||||
class Holder {
|
||||
boost::optional<Dummy> d;
|
||||
|
||||
friend void to_json(nlohmann::json& j, const Holder& h) { /* ... */ }
|
||||
};
|
||||
```
|
||||
|
||||
The [`NLOHMANN_DEFINE_TYPE_INTRUSIVE`](https://json.nlohmann.me/api/macros/nlohmann_define_type_intrusive/index.md) macros define hidden friends as well. See [#3669](https://github.com/nlohmann/json/issues/3669) for details.
|
||||
|
||||
### Missing STL function
|
||||
|
||||
Questions
|
||||
|
||||
Reference in new issue
Block a user