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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 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class value_t {
&lt;&lt;enumeration&gt;&gt;
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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=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/types/number_handling/" rel="canonical"><link href=../ rel=prev><link href=../template_parameters/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Number Handling - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link 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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=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/types/number_handling/" rel="canonical"><link href=../ rel=prev><link href=../template_parameters/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Number Handling - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link 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</a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Number Handling </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)" 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viewbox="0 0 24 24"><path d="M9.5Line truncated
A["number literal"] --&gt; B{{"has a fraction (.) or exponent (e/E)?"}}
B --&gt;|"yes"| F["number_float_t"]
B --&gt;|"no"| C{{"has a leading minus sign?"}}
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@@ -28,15 +28,16 @@ Requirements are split into two groups:
[`BinaryType`](#binarytype)'s `value_type`, and [`NumberUnsignedType`](#numberintegertype-and-numberunsignedtype)
being at least as wide as [`NumberIntegerType`](#numberintegertype-and-numberunsignedtype). The rest are not
diagnosed with dedicated error messages, and violating most of them results in a compiler error somewhere inside
the library. Four violations are not caught at compile time at all:
the library. Four violations are not (always) caught at compile time:
- A [`StringType`](#stringtype) whose `data()` is not null-terminated compiles and silently misparses numbers
stored as a `#!cpp long double` that is not IEEE 754 binary64 (e.g., the 80-bit x87 format), because the lexer
hands the buffer to `#!cpp std::strtold`.
- A stateful [`AllocatorType`](#allocatortype) compiles and silently ignores its state: allocation, deallocation,
and [`get_allocator()`](../../api/basic_json/get_allocator.md) each use a different default-constructed instance.
- The two [cross-specialization conversions](#cross-specialization-conversions) below. These abort on an assertion
in a normal build, and only fail silently under `#!cpp NDEBUG`.
- The two [cross-specialization conversions](#cross-specialization-conversions) below, when the source string type
is a range of characters. They compile and fail the `#!cpp JSON_ASSERT`: with its default definition, the program
aborts unless `#!cpp NDEBUG` is defined, in which case the wrong result is returned silently.
## Overview
@@ -777,7 +778,7 @@ using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;
| Type | Reason |
|----------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------|
| `#!cpp std::wstring`, `#!cpp std::u16string`, `#!cpp std::u32string` | the character type is not one byte wide |
| `#!cpp std::u8string` | one byte wide, but `#!cpp char8_t` is not `#!cpp char`-compatible |
| `#!cpp std::u8string` | one byte wide, but `#!cpp char8_t` is not `#!cpp char`-compatible (as a *value*, it converts to and from a JSON string) |
| `absl::Cord` | no `value_type`, and the storage is not contiguous |
| `QString` | no `append(const char*, size_type)`; its `QChar` is also two bytes wide, though that is never diagnosed |
@@ -1420,14 +1421,18 @@ See [`json_base_class_t`](../../api/basic_json/json_base_class_t.md) for an exam
## Cross-specialization conversions
Converting a value from one `basic_json` specialization into another (see the
[converting constructor](../../api/basic_json/basic_json.md)) imposes two additional requirements that are not
diagnosed at compile time. With assertions enabled they abort on the `#!cpp JSON_ASSERT` at the end of the converting
constructor; under `#!cpp NDEBUG` they fail **silently** at runtime:
[converting constructor](../../api/basic_json/basic_json.md)) imposes two additional requirements:
- The target `string_t` must be directly constructible from the source `string_t`. Otherwise the string is converted to
an array of character codes.
- The target `object_t::key_type` must be directly constructible from the source object's key type. Otherwise the
object is converted to an array of key/value pairs.
- The target `string_t` must be directly constructible from the source `string_t`.
- The target `object_t::key_type` must be directly constructible from the source object's key type.
If a requirement is not met, the result depends on the source string type. If it is a range of characters (it has
`begin()` and `end()`, as `std::string` does), the conversion compiles and then fails the `#!cpp JSON_ASSERT` at the
end of the converting constructor: with its default definition, `assert`, the program aborts unless `#!cpp NDEBUG` is
defined, in which case it **silently** returns an array (of character codes for a string, of key/value pairs for an
object). Otherwise, the conversion does not compile, even though
`#!cpp std::is_constructible` reports `#!cpp true`; for example, when converting a `basic_json` whose `string_t` is the
`alt_string` of the unit tests to `json`.
See [issue #3425](https://github.com/nlohmann/json/issues/3425), [`string_t`](../../api/basic_json/string_t.md), and
[`object_t`](../../api/basic_json/object_t.md).
@@ -1,4 +1,4 @@
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<span class=w> </span><span class=n>basic_json</span><span class=p>,</span><span class=w> </span><span class=c1>// mapped_type</span>
<span class=w> </span><span class=n>default_object_comparator_t</span><span class=p>,</span><span class=w> </span><span class=c1>// key_compare</span>
<span class=w> </span><span class=n>AllocatorType</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=n>pair</span><span class=o>&lt;</span><span class=k>const</span><span class=w> </span><span class=n>StringType</span><span class=p>,</span>
@@ -374,7 +374,7 @@
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>[</span><span class=mi>1</span><span class=p>,</span><span class=mi>2</span><span class=p>,</span><span class=mi>3</span><span class=p>]</span>
<span class=kc>true</span>
</code></pre></div> </details> <h3 id=compatible-containers_1>Compatible containers<a class=headerlink href=#compatible-containers_1 title="Permanent link">&para;</a></h3> <table> <thead> <tr> <th>Container</th> <th>Notes</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span></code> (default)</td> <td></td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>deque</span></code></td> <td>references survive appends, but not insertions elsewhere; see the <code>capacity()</code> note above</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>pmr</span><span class=o>::</span><span class=n>vector</span></code></td> <td>through an alias, as the allocator comes from <code>AllocatorType</code> instead</td> </tr> <tr> <td><code>boost::container::vector</code>, <code>deque</code>, <code>devector</code></td> <td></td> </tr> <tr> <td><code>boost::container::stable_vector</code></td> <td>the only one tried that keeps references valid across <em>every</em> insertion</td> </tr> <tr> <td><code>boost::container::small_vector</code>, <code>folly::small_vector</code></td> <td>through an alias that fixes the inline capacity</td> </tr> <tr> <td><code>boost::container::static_vector</code></td> <td>through the same kind of alias, for arrays that stay within the fixed capacity</td> </tr> <tr> <td><code>folly::fbvector</code></td> <td>requires C++20, see the note above</td> </tr> </tbody> </table> <h3 id=containers-that-cannot-be-used_1>Containers that cannot be used<a class=headerlink href=#containers-that-cannot-be-used_1 title="Permanent link">&para;</a></h3> <table> <thead> <tr> <th>Container</th> <th>Reason</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>list</span></code></td> <td>no <code>operator[]</code>, and no random-access iterators</td> </tr> <tr> <td><code>eastl::vector</code>, <code>QList</code>, <code>QVector</code></td> <td>no <code>max_size()</code>; they handle the incomplete value type fine</td> </tr> <tr> <td><code>absl::InlinedVector</code></td> <td>requires a complete value type, see the note above</td> </tr> <tr> <td><code>absl::FixedArray</code></td> <td>the size is fixed at construction, so <code>resize</code>, <code>push_back</code>, <code>insert</code> and <code>erase</code> are missing</td> </tr> </tbody> </table> <h2 id=stringtype><code>StringType</code><a class=headerlink href=#stringtype title="Permanent link">&para;</a></h2> <p><code>StringType</code> is used <strong>both</strong> for <abbr title="JavaScript Object Notation">JSON</abbr> string values and for the keys of <abbr title="JavaScript Object Notation">JSON</abbr> objects (<code>string_t</code> and <code>object_t::key_type</code>).</p> <h3 id=always-required_2>Always required<a class=headerlink href=#always-required_2 title="Permanent link">&para;</a></h3> <ul> <li>A member type <code>value_type</code> that is one byte wide and <code>char</code>-compatible. The library stores and processes <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoded <code>char</code> data and passes <code>data()</code> to functions that take a <code class=highlight><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*</span></code>, such as <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>strtold</span></code> (only used to parse a <code class=highlight><span class=kt>long</span><span class=w> </span><span class=kt>double</span></code> that is not <abbr title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 754 binary64, see <a href=#numberfloattype><code>NumberFloatType</code></a>). <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>wstring</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u16string</span></code>, and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u32string</span></code> are <strong>not</strong> valid choices; see the <abbr title="Frequently Asked Questions">FAQ</abbr> on <a href=../../../home/faq/#wide-string-handling>wide string handling</a>.</li> <li>Constructors: default, copy, move, from <code class=highlight><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*</span></code> (which must not be <code class=highlight><span class=k>explicit</span></code>), from <code class=highlight><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*</span><span class=p>,</span><span class=w> </span><span class=n>size_type</span><span class=p>)</span></code>, and from <code class=highlight><span class=p>(</span><span class=n>size_type</span><span class=p>,</span><span class=w> </span><span class=kt>char</span><span Line truncated
</code></pre></div> </details> <h3 id=compatible-containers_1>Compatible containers<a class=headerlink href=#compatible-containers_1 title="Permanent link">&para;</a></h3> <table> <thead> <tr> <th>Container</th> <th>Notes</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span></code> (default)</td> <td></td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>deque</span></code></td> <td>references survive appends, but not insertions elsewhere; see the <code>capacity()</code> note above</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>pmr</span><span class=o>::</span><span class=n>vector</span></code></td> <td>through an alias, as the allocator comes from <code>AllocatorType</code> instead</td> </tr> <tr> <td><code>boost::container::vector</code>, <code>deque</code>, <code>devector</code></td> <td></td> </tr> <tr> <td><code>boost::container::stable_vector</code></td> <td>the only one tried that keeps references valid across <em>every</em> insertion</td> </tr> <tr> <td><code>boost::container::small_vector</code>, <code>folly::small_vector</code></td> <td>through an alias that fixes the inline capacity</td> </tr> <tr> <td><code>boost::container::static_vector</code></td> <td>through the same kind of alias, for arrays that stay within the fixed capacity</td> </tr> <tr> <td><code>folly::fbvector</code></td> <td>requires C++20, see the note above</td> </tr> </tbody> </table> <h3 id=containers-that-cannot-be-used_1>Containers that cannot be used<a class=headerlink href=#containers-that-cannot-be-used_1 title="Permanent link">&para;</a></h3> <table> <thead> <tr> <th>Container</th> <th>Reason</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>list</span></code></td> <td>no <code>operator[]</code>, and no random-access iterators</td> </tr> <tr> <td><code>eastl::vector</code>, <code>QList</code>, <code>QVector</code></td> <td>no <code>max_size()</code>; they handle the incomplete value type fine</td> </tr> <tr> <td><code>absl::InlinedVector</code></td> <td>requires a complete value type, see the note above</td> </tr> <tr> <td><code>absl::FixedArray</code></td> <td>the size is fixed at construction, so <code>resize</code>, <code>push_back</code>, <code>insert</code> and <code>erase</code> are missing</td> </tr> </tbody> </table> <h2 id=stringtype><code>StringType</code><a class=headerlink href=#stringtype title="Permanent link">&para;</a></h2> <p><code>StringType</code> is used <strong>both</strong> for <abbr title="JavaScript Object Notation">JSON</abbr> string values and for the keys of <abbr title="JavaScript Object Notation">JSON</abbr> objects (<code>string_t</code> and <code>object_t::key_type</code>).</p> <h3 id=always-required_2>Always required<a class=headerlink href=#always-required_2 title="Permanent link">&para;</a></h3> <ul> <li>A member type <code>value_type</code> that is one byte wide and <code>char</code>-compatible. The library stores and processes <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoded <code>char</code> data and passes <code>data()</code> to functions that take a <code class=highlight><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*</span></code>, such as <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>strtold</span></code> (only used to parse a <code class=highlight><span class=kt>long</span><span class=w> </span><span class=kt>double</span></code> that is not <abbr title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 754 binary64, see <a href=#numberfloattype><code>NumberFloatType</code></a>). <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>wstring</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u16string</span></code>, and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u32string</span></code> are <strong>not</strong> valid choices; see the <abbr title="Frequently Asked Questions">FAQ</abbr> on <a href=../../../home/faq/#wide-string-handling>wide string handling</a>.</li> <li>Constructors: default, copy, move, from <code class=highlight><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*</span></code> (which must not be <code class=highlight><span class=k>explicit</span></code>), from <code class=highlight><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*</span><span class=p>,</span><span class=w> </span><span class=n>size_type</span><span class=p>)</span></code>, and from <code class=highlight><span class=p>(</span><span class=n>size_type</span><span class=p>,</span><span class=w> </span><span class=kt>char</span><span Line truncated
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;cstddef&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;ostream&gt;</span>
@@ -678,4 +678,4 @@
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=nt>"bytes"</span><span class=p>:[</span><span class=mi>1</span><span class=p>,</span><span class=mi>2</span><span class=p>,</span><span class=mi>3</span><span class=p>],</span><span class=nt>"subtype"</span><span class=p>:</span><span class=kc>null</span><span class=p>}</span>
<span class=kc>true</span>
</code></pre></div> </details> <h3 id=compatible-containers_2>Compatible containers<a class=headerlink href=#compatible-containers_2 title="Permanent link">&para;</a></h3> <table> <thead> <tr> <th>Container</th> <th>Notes</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span></code> (default)</td> <td></td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=kt>char</span><span class=o>&gt;</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>byte</span><span class=o>&gt;</span></code></td> <td><code>dump()</code> writes the bytes as 0..255 whichever is used</td> </tr> <tr> <td><code>boost::container::vector&lt;std::uint8_t&gt;</code>, <code>boost::container::small_vector&lt;std::uint8_t, N&gt;</code></td> <td></td> </tr> <tr> <td><code>absl::InlinedVector&lt;std::uint8_t, N&gt;</code></td> <td>usable here, unlike as an <code>ArrayType</code>, because the value type is complete</td> </tr> <tr> <td><code>eastl::vector&lt;std::uint8_t&gt;</code></td> <td>usable here, unlike as an <code>ArrayType</code>, because <code>max_size()</code> is not needed</td> </tr> <tr> <td><code>folly::fbvector&lt;std::uint8_t&gt;</code></td> <td>requires C++20, see the note above</td> </tr> </tbody> </table> <h3 id=containers-that-cannot-be-used_2>Containers that cannot be used<a class=headerlink href=#containers-that-cannot-be-used_2 title="Permanent link">&para;</a></h3> <table> <thead> <tr> <th>Container</th> <th>Reason</th> </tr> </thead> <tbody> <tr> <td><code>QByteArray</code></td> <td>no <code>empty()</code> (it spells that <code>isEmpty()</code>); its <code>insert</code> takes an index rather than an iterator; and it converts to <code>string_t</code>, which makes <code>to_json</code> ambiguous between a string and a binary value</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code></td> <td><code>binary_t::container_type</code> and <code>string_t</code> would be the same type, so the two <a href=../../../api/basic_json/swap/ ><code>swap</code></a> overloads collide and <code>basic_json</code> cannot be instantiated at all</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>deque</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span></code></td> <td>storage is not contiguous, so there is no <code>data()</code></td> </tr> <tr> <td>containers whose <code>value_type</code> is wider than one byte</td> <td>see above — accepted by the compiler, wrong at runtime</td> </tr> </tbody> </table> <h2 id=custombaseclass><code>CustomBaseClass</code><a class=headerlink href=#custombaseclass title="Permanent link">&para;</a></h2> <p><code>CustomBaseClass</code> is an extension point: unless it is <code class=highlight><span class=kt>void</span></code> (the default, which selects the empty <code>nlohmann::json_default_base</code>), <code>basic_json</code> publicly derives from it.</p> <h3 id=always-required_9>Always required<a class=headerlink href=#always-required_9 title="Permanent link">&para;</a></h3> <ul> <li>A non-<code>final</code>, default-constructible class type.</li> <li><code>basic_json</code> is copy-/move-constructible and copy-/move-assignable only if <code>CustomBaseClass</code> is.</li> </ul> <h3 id=notes_2>Notes<a class=headerlink href=#notes_2 title="Permanent link">&para;</a></h3> <p><code>basic_json</code> is documented to be a <a href="https://en.cppreference.com/w/cpp/named_req/StandardLayoutType">StandardLayoutType</a>. Because <code>basic_json</code> has non-static data members of its own, a <code>CustomBaseClass</code> with non-static data members forfeits this guarantee.</p> <p>Note the namespace of <code>CustomBaseClass</code> becomes an associated namespace of <code>basic_json</code> for the purpose of argument-dependent lookup.</p> <p>See <a href=../../../api/basic_json/json_base_class_t/ ><code>json_base_class_t</code></a> for an example.</p> <h3 id=compatible-types_6>Compatible types<a class=headerlink href=#compatible-types_6 title="Permanent link">&para;</a></h3> <table> <thead> <tr> <th>Type</th> <th>Support</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=kt>void</span></code> (default)</td> <td>an empty base class is used; no effect on <code>basic_json</code></td> </tr> <tr> <td>any default-constructible, non-<code>final</code> class</td> <td>full; see <a href=../../../api/basic_json/json_base_class_t/ ><code>json_base_class_t</code></a></td> </trLine truncated
</code></pre></div> </details> <h3 id=compatible-containers_2>Compatible containers<a class=headerlink href=#compatible-containers_2 title="Permanent link">&para;</a></h3> <table> <thead> <tr> <th>Container</th> <th>Notes</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span></code> (default)</td> <td></td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=kt>char</span><span class=o>&gt;</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>byte</span><span class=o>&gt;</span></code></td> <td><code>dump()</code> writes the bytes as 0..255 whichever is used</td> </tr> <tr> <td><code>boost::container::vector&lt;std::uint8_t&gt;</code>, <code>boost::container::small_vector&lt;std::uint8_t, N&gt;</code></td> <td></td> </tr> <tr> <td><code>absl::InlinedVector&lt;std::uint8_t, N&gt;</code></td> <td>usable here, unlike as an <code>ArrayType</code>, because the value type is complete</td> </tr> <tr> <td><code>eastl::vector&lt;std::uint8_t&gt;</code></td> <td>usable here, unlike as an <code>ArrayType</code>, because <code>max_size()</code> is not needed</td> </tr> <tr> <td><code>folly::fbvector&lt;std::uint8_t&gt;</code></td> <td>requires C++20, see the note above</td> </tr> </tbody> </table> <h3 id=containers-that-cannot-be-used_2>Containers that cannot be used<a class=headerlink href=#containers-that-cannot-be-used_2 title="Permanent link">&para;</a></h3> <table> <thead> <tr> <th>Container</th> <th>Reason</th> </tr> </thead> <tbody> <tr> <td><code>QByteArray</code></td> <td>no <code>empty()</code> (it spells that <code>isEmpty()</code>); its <code>insert</code> takes an index rather than an iterator; and it converts to <code>string_t</code>, which makes <code>to_json</code> ambiguous between a string and a binary value</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code></td> <td><code>binary_t::container_type</code> and <code>string_t</code> would be the same type, so the two <a href=../../../api/basic_json/swap/ ><code>swap</code></a> overloads collide and <code>basic_json</code> cannot be instantiated at all</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>deque</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span></code></td> <td>storage is not contiguous, so there is no <code>data()</code></td> </tr> <tr> <td>containers whose <code>value_type</code> is wider than one byte</td> <td>see above — accepted by the compiler, wrong at runtime</td> </tr> </tbody> </table> <h2 id=custombaseclass><code>CustomBaseClass</code><a class=headerlink href=#custombaseclass title="Permanent link">&para;</a></h2> <p><code>CustomBaseClass</code> is an extension point: unless it is <code class=highlight><span class=kt>void</span></code> (the default, which selects the empty <code>nlohmann::json_default_base</code>), <code>basic_json</code> publicly derives from it.</p> <h3 id=always-required_9>Always required<a class=headerlink href=#always-required_9 title="Permanent link">&para;</a></h3> <ul> <li>A non-<code>final</code>, default-constructible class type.</li> <li><code>basic_json</code> is copy-/move-constructible and copy-/move-assignable only if <code>CustomBaseClass</code> is.</li> </ul> <h3 id=notes_2>Notes<a class=headerlink href=#notes_2 title="Permanent link">&para;</a></h3> <p><code>basic_json</code> is documented to be a <a href="https://en.cppreference.com/w/cpp/named_req/StandardLayoutType">StandardLayoutType</a>. Because <code>basic_json</code> has non-static data members of its own, a <code>CustomBaseClass</code> with non-static data members forfeits this guarantee.</p> <p>Note the namespace of <code>CustomBaseClass</code> becomes an associated namespace of <code>basic_json</code> for the purpose of argument-dependent lookup.</p> <p>See <a href=../../../api/basic_json/json_base_class_t/ ><code>json_base_class_t</code></a> for an example.</p> <h3 id=compatible-types_6>Compatible types<a class=headerlink href=#compatible-types_6 title="Permanent link">&para;</a></h3> <table> <thead> <tr> <th>Type</th> <th>Support</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=kt>void</span></code> (default)</td> <td>an empty base class is used; no effect on <code>basic_json</code></td> </tr> <tr> <td>any default-constructible, non-<code>final</code> class</td> <td>full; see <a href=../../../api/basic_json/json_base_class_t/ ><code>json_base_class_t</code></a></td> </trLine truncated
+13 -11
View File
@@ -13,11 +13,11 @@ Requirements are split into two groups:
Requirements are not checked
Three requirements are checked with a `static_assert`: the array iterator category, the width of [`BinaryType`](#binarytype)'s `value_type`, and [`NumberUnsignedType`](#numberintegertype-and-numberunsignedtype) being at least as wide as [`NumberIntegerType`](#numberintegertype-and-numberunsignedtype). The rest are not diagnosed with dedicated error messages, and violating most of them results in a compiler error somewhere inside the library. Four violations are not caught at compile time at all:
Three requirements are checked with a `static_assert`: the array iterator category, the width of [`BinaryType`](#binarytype)'s `value_type`, and [`NumberUnsignedType`](#numberintegertype-and-numberunsignedtype) being at least as wide as [`NumberIntegerType`](#numberintegertype-and-numberunsignedtype). The rest are not diagnosed with dedicated error messages, and violating most of them results in a compiler error somewhere inside the library. Four violations are not (always) caught at compile time:
- A [`StringType`](#stringtype) whose `data()` is not null-terminated compiles and silently misparses numbers stored as a `long double` that is not IEEE 754 binary64 (e.g., the 80-bit x87 format), because the lexer hands the buffer to `std::strtold`.
- A stateful [`AllocatorType`](#allocatortype) compiles and silently ignores its state: allocation, deallocation, and [`get_allocator()`](https://json.nlohmann.me/api/basic_json/get_allocator/index.md) each use a different default-constructed instance.
- The two [cross-specialization conversions](#cross-specialization-conversions) below. These abort on an assertion in a normal build, and only fail silently under `NDEBUG`.
- The two [cross-specialization conversions](#cross-specialization-conversions) below, when the source string type is a range of characters. They compile and fail the `JSON_ASSERT`: with its default definition, the program aborts unless `NDEBUG` is defined, in which case the wrong result is returned silently.
## Overview
@@ -655,12 +655,12 @@ true
### Types that cannot be used
| Type | Reason |
| -------------------------------------------------- | ------------------------------------------------------------------------------------------------------- |
| `std::wstring`, `std::u16string`, `std::u32string` | the character type is not one byte wide |
| `std::u8string` | one byte wide, but `char8_t` is not `char`-compatible |
| `absl::Cord` | no `value_type`, and the storage is not contiguous |
| `QString` | no `append(const char*, size_type)`; its `QChar` is also two bytes wide, though that is never diagnosed |
| Type | Reason |
| -------------------------------------------------- | ----------------------------------------------------------------------------------------------------------- |
| `std::wstring`, `std::u16string`, `std::u32string` | the character type is not one byte wide |
| `std::u8string` | one byte wide, but `char8_t` is not `char`-compatible (as a *value*, it converts to and from a JSON string) |
| `absl::Cord` | no `value_type`, and the storage is not contiguous |
| `QString` | no `append(const char*, size_type)`; its `QChar` is also two bytes wide, though that is never diagnosed |
A `std::pmr::string` mostly does not use the memory resource you choose
@@ -1208,10 +1208,12 @@ See [`json_base_class_t`](https://json.nlohmann.me/api/basic_json/json_base_clas
## Cross-specialization conversions
Converting a value from one `basic_json` specialization into another (see the [converting constructor](https://json.nlohmann.me/api/basic_json/basic_json/index.md)) imposes two additional requirements that are not diagnosed at compile time. With assertions enabled they abort on the `JSON_ASSERT` at the end of the converting constructor; under `NDEBUG` they fail **silently** at runtime:
Converting a value from one `basic_json` specialization into another (see the [converting constructor](https://json.nlohmann.me/api/basic_json/basic_json/index.md)) imposes two additional requirements:
- The target `string_t` must be directly constructible from the source `string_t`. Otherwise the string is converted to an array of character codes.
- The target `object_t::key_type` must be directly constructible from the source object's key type. Otherwise the object is converted to an array of key/value pairs.
- The target `string_t` must be directly constructible from the source `string_t`.
- The target `object_t::key_type` must be directly constructible from the source object's key type.
If a requirement is not met, the result depends on the source string type. If it is a range of characters (it has `begin()` and `end()`, as `std::string` does), the conversion compiles and then fails the `JSON_ASSERT` at the end of the converting constructor: with its default definition, `assert`, the program aborts unless `NDEBUG` is defined, in which case it **silently** returns an array (of character codes for a string, of key/value pairs for an object). Otherwise, the conversion does not compile, even though `std::is_constructible` reports `true`; for example, when converting a `basic_json` whose `string_t` is the `alt_string` of the unit tests to `json`.
See [issue #3425](https://github.com/nlohmann/json/issues/3425), [`string_t`](https://json.nlohmann.me/api/basic_json/string_t/index.md), and [`object_t`](https://json.nlohmann.me/api/basic_json/object_t/index.md).