deploy: 4e294f4b18

home/faq/index.html

@@ -8,7 +8,7 @@
 
 <span class=n>json</span><span class=w> </span><span class=n>obj</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{{</span><span class=s>"key"</span><span class=p>,</span><span class=w> </span><span class=s>"value"</span><span class=p>}};</span>
 <span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>{</span><span class=n>obj</span><span class=p>};</span><span class=w>   </span><span class=c1>// -&gt; {"key":"value"}  (copy, not array)</span>
-</code></pre></div> <p>Without the macro (default behavior), <code>json j{obj}</code> creates <code>[{"key":"value"}]</code>. This opt-in macro fixes issue #5074 while preserving backwards compatibility for existing code.</p> <h2 id=limitations>Limitations<a class=headerlink href=#limitations title="Permanent link">&para;</a></h2> <h3 id=relaxed-parsing>Relaxed parsing<a class=headerlink href=#relaxed-parsing title="Permanent link">&para;</a></h3> <div class="admonition question"> <p class=admonition-title>Question</p> <p>Can you add an option to ignore trailing commas?</p> </div> <p>This library does not support any feature that would jeopardize interoperability.</p> <h3 id=parse-errors-reading-non-ascii-characters>Parse errors reading non-<abbr title="American Standard Code for Information Interchange">ASCII</abbr> characters<a class=headerlink href=#parse-errors-reading-non-ascii-characters title="Permanent link">&para;</a></h3> <div class="admonition question"> <p class=admonition-title>Questions</p> <ul> <li>Why is the parser complaining about a Chinese character?</li> <li>Does the library support Unicode?</li> <li>I get an exception <code>[json.exception.parse_error.101] parse error at line 1, column 53: syntax error while parsing value - invalid string: ill-formed UTF-8 byte; last read: '"Testé$')"</code></li> </ul> </div> <p>The library supports <strong>Unicode input</strong> as follows:</p> <ul> <li>Only <strong><abbr title="Unicode Transformation Format">UTF</abbr>-8</strong> encoded input is supported, which is the default encoding for <abbr title="JavaScript Object Notation">JSON</abbr>, according to <a href="https://tools.ietf.org/html/rfc8259.html#section-8.1"><abbr title="Request for Comments">RFC</abbr> 8259</a>.</li> <li><code>std::u16string</code> and <code>std::u32string</code> can be parsed, assuming <abbr title="Unicode Transformation Format">UTF</abbr>-16 and <abbr title="Unicode Transformation Format">UTF</abbr>-32 encoding, respectively. These encodings are not supported when reading from files or other input containers.</li> <li>Other encodings such as Latin-1 or <abbr title="International Organization for Standardization">ISO</abbr> 8859-1 are <strong>not</strong> supported and will yield parse or serialization errors.</li> <li>The library will not replace <a href="http://www.unicode.org/faq/private_use.html#nonchar1">Unicode noncharacters</a>.</li> <li>Invalid surrogates (e.g., incomplete pairs such as <code>\uDEAD</code>) will yield parse errors.</li> <li>The strings stored in the library are <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoded. When using the default string type (<code>std::string</code>), note that its length/size functions return the number of stored bytes rather than the number of characters or glyphs.</li> <li>When you store strings with different encodings in the library, calling <a href=../../api/basic_json/dump/ ><code>dump()</code></a> may throw an exception unless <code>json::error_handler_t::replace</code>, <code>json::error_handler_t::ignore</code>, or <code>json::error_handler_t::keep</code> are used as error handlers.</li> </ul> <p>In most cases, the parser is right to complain, because the input is not <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoded. This is especially true for Microsoft Windows, where Latin-1 or <abbr title="International Organization for Standardization">ISO</abbr> 8859-1 is often the standard encoding.</p> <h3 id=nul-bytes-in-the-input>NUL bytes in the input<a class=headerlink href=#nul-bytes-in-the-input title="Permanent link">&para;</a></h3> <div class="admonition question"> <p class=admonition-title>Questions</p> <ul> <li>Why does <a href=../../api/basic_json/parse/ ><code>json::parse()</code></a> silently ignore part of my input?</li> <li>Why does a <code>std::string</code>/buffer with extra data after the <abbr title="JavaScript Object Notation">JSON</abbr> text parse without error, while a similar-looking string with extra text does not?</li> </ul> </div> <p>A <code>'\0'</code> (NUL) byte anywhere in the input is treated the same as the real end of the input, rather than as an ordinary (and, outside of a string, invalid) byte. Everything from that byte onward is silently ignored, without a parse error — including further, otherwise well-formed <abbr title="JavaScript Object Notation">JSON</abbr>:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=p>(</span><span class=s>"123"</span><span class=p>)</span><span class=w> </span><span class=o>+</span><span class=w> </span><span class=sc>'\0'</span><span class=p>);</span><span class=w>          </span><span class=c1>// == 123, no error</span>
+</code></pre></div> <p>Without the macro (default behavior), <code>json j{obj}</code> creates <code>[{"key":"value"}]</code>. This opt-in macro fixes issue #5074 while preserving backwards compatibility for existing code.</p> <h2 id=limitations>Limitations<a class=headerlink href=#limitations title="Permanent link">&para;</a></h2> <h3 id=relaxed-parsing>Relaxed parsing<a class=headerlink href=#relaxed-parsing title="Permanent link">&para;</a></h3> <div class="admonition question"> <p class=admonition-title>Question</p> <p>Can you add an option to ignore trailing commas?</p> </div> <p>Yes, trailing commas in arrays and objects can be allowed by setting parameter <code>ignore_trailing_commas</code> of <a href=../../api/basic_json/parse/ ><code>parse</code></a> to <code class=highlight><span class=nb>true</span></code>; see <a href=../../features/trailing_commas/ >trailing commas</a>. They are rejected by default, and the library never adds them when serializing.</p> <h3 id=parse-errors-reading-non-ascii-characters>Parse errors reading non-<abbr title="American Standard Code for Information Interchange">ASCII</abbr> characters<a class=headerlink href=#parse-errors-reading-non-ascii-characters title="Permanent link">&para;</a></h3> <div class="admonition question"> <p class=admonition-title>Questions</p> <ul> <li>Why is the parser complaining about a Chinese character?</li> <li>Does the library support Unicode?</li> <li>I get an exception <code>[json.exception.parse_error.101] parse error at line 1, column 53: syntax error while parsing value - invalid string: ill-formed UTF-8 byte; last read: '"Testé$')"</code></li> </ul> </div> <p>The library supports <strong>Unicode input</strong> as follows:</p> <ul> <li>Only <strong><abbr title="Unicode Transformation Format">UTF</abbr>-8</strong> encoded input is supported, which is the default encoding for <abbr title="JavaScript Object Notation">JSON</abbr>, according to <a href="https://tools.ietf.org/html/rfc8259.html#section-8.1"><abbr title="Request for Comments">RFC</abbr> 8259</a>.</li> <li><code>std::u16string</code> and <code>std::u32string</code> can be parsed, assuming <abbr title="Unicode Transformation Format">UTF</abbr>-16 and <abbr title="Unicode Transformation Format">UTF</abbr>-32 encoding, respectively. These encodings are not supported when reading from files or other input containers.</li> <li>Other encodings such as Latin-1 or <abbr title="International Organization for Standardization">ISO</abbr> 8859-1 are <strong>not</strong> supported and will yield parse or serialization errors.</li> <li>The library will not replace <a href="http://www.unicode.org/faq/private_use.html#nonchar1">Unicode noncharacters</a>.</li> <li>Invalid surrogates (e.g., incomplete pairs such as <code>\uDEAD</code>) will yield parse errors.</li> <li>The strings stored in the library are <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoded. When using the default string type (<code>std::string</code>), note that its length/size functions return the number of stored bytes rather than the number of characters or glyphs.</li> <li>When you store strings with different encodings in the library, calling <a href=../../api/basic_json/dump/ ><code>dump()</code></a> may throw an exception unless <code>json::error_handler_t::replace</code>, <code>json::error_handler_t::ignore</code>, or <code>json::error_handler_t::keep</code> are used as error handlers.</li> </ul> <p>In most cases, the parser is right to complain, because the input is not <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoded. This is especially true for Microsoft Windows, where Latin-1 or <abbr title="International Organization for Standardization">ISO</abbr> 8859-1 is often the standard encoding.</p> <h3 id=nul-bytes-in-the-input>NUL bytes in the input<a class=headerlink href=#nul-bytes-in-the-input title="Permanent link">&para;</a></h3> <div class="admonition question"> <p class=admonition-title>Questions</p> <ul> <li>Why does <a href=../../api/basic_json/parse/ ><code>json::parse()</code></a> silently ignore part of my input?</li> <li>Why does a <code>std::string</code>/buffer with extra data after the <abbr title="JavaScript Object Notation">JSON</abbr> text parse without error, while a similar-looking string with extra text does not?</li> </ul> </div> <p>A <code>'\0'</code> (NUL) byte anywhere in the input is treated the same as the real end of the input, rather than as an ordinary (and, outside of a string, invalid) byte. Everything from that byte onward is silently ignored, without a parse error — including further, otherwise well-formed <abbr title="JavaScript Object Notation">JSON</abbr>:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=p>(</span><span class=s>"123"</span><span class=p>)</span><span class=w>Line truncated
 <span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=p>(</span><span class=s>"123"</span><span class=p>)</span><span class=w> </span><span class=o>+</span><span class=w> </span><span class=sc>'\0'</span><span class=w> </span><span class=o>+</span><span class=w> </span><span class=s>"true"</span><span class=p>);</span><span class=w> </span><span class=c1>// == 123, the "true" is silently ignored too</span>
 </code></pre></div> <p>This is different from any other unexpected trailing byte, which <em>does</em> raise <a href=../exceptions/#jsonexceptionparse_error101><code>parse_error.101</code></a>:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=s>"123x"</span><span class=p>);</span><span class=w> </span><span class=c1>// throws parse_error.101: unexpected additional data</span>
 </code></pre></div> <p>This falls out of the same convention used when no explicit input length is given at all: <code>json::parse(const char*)</code> already stops at the first NUL byte via <code>strlen()</code>, since a bare pointer has no length of its own. The library applies that same NUL-terminated-C-string convention uniformly, rather than only when a length is genuinely unavailable — so a <code>std::string</code>, iterator range, or container whose content happens to include a NUL byte is affected the same way a raw <code>const char*</code> would be.</p> <p>If your input may contain a trailing or embedded NUL that is <strong>not</strong> meant to signal the end of the <abbr title="JavaScript Object Notation">JSON</abbr> text — for instance, a fixed-size, zero-padded buffer — trim it yourself before calling <code>parse()</code>, since the library will otherwise silently stop there instead of raising an error:</p> <div class=highlight><pre><span></span><code><span class=n>s</span><span class=p>.</span><span class=n>resize</span><span class=p>(</span><span class=n>s</span><span class=p>.</span><span class=n>find</span><span class=p>(</span><span class=sc>'\0'</span><span class=p>));</span><span class=w> </span><span class=c1>// drop everything from the first NUL onward, if any</span>
@@ -39,7 +39,7 @@
 <span class=w>  </span><span class=nt>"encoded"</span><span class=p>:</span><span class=w> </span><span class=s2>"車B1234 こんにちは"</span><span class=p>,</span>
 <span class=w>  </span><span class=nt>"original"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=mi>36554</span><span class=p>,</span><span class=w> </span><span class=mi>66</span><span class=p>,</span><span class=w> </span><span class=mi>49</span><span class=p>,</span><span class=w> </span><span class=mi>50</span><span class=p>,</span><span class=w> </span><span class=mi>51</span><span class=p>,</span><span class=w> </span><span class=mi>52</span><span class=p>,</span><span class=w> </span><span class=mi>32</span><span class=p>,</span><span class=w> </span><span class=mi>12371</span><span class=p>,</span><span class=w> </span><span class=mi>12435</span><span class=p>,</span><span class=w> </span><span class=mi>12395</span><span class=p>,</span><span class=w> </span><span class=mi>12385</span><span class=p>,</span><span class=w> </span><span class=mi>12399</span><span class=p>]</span>
 <span class=p>}</span>
-</code></pre></div> </div> <h2 id=usage>Usage<a class=headerlink href=#usage title="Permanent link">&para;</a></h2> <h3 id=thread-safety>Thread safety<a class=headerlink href=#thread-safety title="Permanent link">&para;</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">&para;</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">&para;</a></h2> <h3 id=parsing-without-exceptions>Parsing without exceptions<a class=headerlink href=#parsing-without-exceptions title="Permanent link">&para;</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">&para;</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">&para;</a></h2> <h3 id=number-precision>Number precision<a class=headerlink href=#number-precision title="Permanent link">&para;</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&lt;number_float_t&gt;::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 -&gt; value -&gt; 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&lt;T&gt;::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">&para;</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
+</code></pre></div> </div> <h2 id=usage>Usage<a class=headerlink href=#usage title="Permanent link">&para;</a></h2> <h3 id=thread-safety>Thread safety<a class=headerlink href=#thread-safety title="Permanent link">&para;</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">&para;</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">&para;</a></h2> <h3 id=parsing-without-exceptions>Parsing without exceptions<a class=headerlink href=#parsing-without-exceptions title="Permanent link">&para;</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">&para;</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">&para;</a></h2> <h3 id=number-precision>Number precision<a class=headerlink href=#number-precision title="Permanent link">&para;</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 serializes floating-point numbers with the shortest representation that parses back to the same value, using at most <code>std::numeric_limits&lt;number_float_t&gt;::max_digits10</code> (17 for <abbr title="Institute of Electrical and Electronics Engineers">IEEE</abbr> <code>double</code>s) significant digits. This guarantees that value -&gt; string -&gt; value round-trips. The reverse direction, string -&gt; value -&gt; string, is only guaranteed for numbers with at most <code>std::numeric_limits&lt;number_float_t&gt;::digits10</code> (15 for <abbr title="Institute of Electrical and Electronics Engineers">IEEE</abbr> <code>double</code>s) significant digits; a number with more digits may not be representable exactly and is then serialized as the closest <code>double</code>.</p> <div class="admonition quote"> <p class=admonition-title><a href="https://en.cppreference.com/w/cpp/types/numeric_limits/max_digits10">cppreference.com</a></p> <p>The value of <code>std::numeric_limits&lt;T&gt;::max_digits10</code> is the number of base-10 digits that are necessary to uniquely represent all distinct values of the type T, such as necessary for serialization/deserialization to text.</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">&para;</a></h3> <div class="admonition question"> <p class=admonition-title>Questions</p> <ul> <li>Why does <code>dump()</code> throw when I serializLine truncated
 <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">&para;</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>&lt;format&gt;</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&lt;basic_json&gt;</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>"{:.&gt;#}"</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&lt;basic_json&gt;</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&lt;json&gt;</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">&para;</a></h2> <h3 id=android-sdk>Android <abbr title="Software Development Kit">SDK</abbr><a class=headerlink href=#android-sdk title="Permanent link">&para;</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> &lt; 11<a class=headerlink href=#incomplete-detector-type-with-gcc-11 title="Permanent link">&para;</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&lt;..., to_json_function, ...&gt;'</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&lt;Dummy&gt;</code> member (e.g., <code>boost::optiLine truncated
 <span class=w>  </span><span class=k>public</span><span class=o>:</span>
@@ -56,4 +56,4 @@
 
 <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>&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>Holder</span><span class=o>&amp;</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">&para;</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 7, 2026 17:17:57 UTC">October 7, 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, CodLine truncated
+</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">&para;</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 14, 2026 15:40:55 UTC">October 14, 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, CLine truncated