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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/api/basic_json_view/count/" rel="canonical"><link href=../contains/ rel=prev><link href=../dump/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>count - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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<span class=n>size_type</span><span class=w> </span><span class=nf>count</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*</span><span class=w> </span><span class=n>key</span><span class=p>)</span><span class=w> </span><span class=k>const</span><span class=p>;</span>
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<span class=n>size_type</span><span class=w> </span><span class=nf>count</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>string_t</span><span class=o>&</span><span class=w> </span><span class=n>key</span><span class=p>)</span><span class=w> </span><span class=k>const</span><span class=p>;</span>
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</code></pre></div> <p>Returns the number of members with key <code>key</code> if the value is an object, <code class=highlight><span class=mi>0</span></code> otherwise. Members with a repeated key are all counted.</p> <h2 id=parameters>Parameters<a class=headerlink href=#parameters title="Permanent link">¶</a></h2> <dl> <dt><code>key</code> (in)</dt> <dd>key value of the element to count</dd> </dl> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">¶</a></h2> <p>the number of members of the object with key <code>key</code> (duplicates included), or <code class=highlight><span class=mi>0</span></code> if the value is not an object or has no such member.</p> <h2 id=exception-safety>Exception safety<a class=headerlink href=#exception-safety title="Permanent link">¶</a></h2> <p>No-throw guarantee: this function never throws exceptions.</p> <h2 id=complexity>Complexity<a class=headerlink href=#complexity title="Permanent link">¶</a></h2> <p>Linear in the number of members: unlike the lookup functions, <code>count()</code> does not stop at the first match, because it has to count every member with the key. Members are compared one after another, in document order. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content. The hash index of the keys of objects with 128 or more members does not help here, since every member is visited.</p> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <p>This method always returns <code class=highlight><span class=mi>0</span></code> when the value is not an object -- including a <a href=../is_discarded/ >discarded</a> view.</p> <p>If the source text repeats a key, <code>count()</code> returns more than <code class=highlight><span class=mi>1</span></code> for it, even though the <a href=../materialize/ >materialized</a> value (like the result of <code>parse()</code>) would have only one member with that key. Hence <code class=highlight><span class=n>count</span><span class=p>(</span><span class=n>key</span><span class=p>)</span><span class=w> </span><span class=o>></span><span class=w> </span><span class=mi>1</span></code> detects duplicate keys. This is unlike the lookup functions (<a href=../operator%5B%5D/ ><code>operator[]</code></a>, <a href=../at/ ><code>at</code></a>, <a href=../find/ ><code>find</code></a>, <a href=../contains/ ><code>contains</code></a>), which resolve to the <em>first</em> member; see the <a href=../operator%5B%5D/#notes>Notes on duplicate keys</a> of <code>operator[]</code>.</p> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">¶</a></h2> <details class=example> <summary>Example</summary> <p>The example below validates that every transaction of a batch carries exactly one <code>amount</code> field, using <code>count()</code> before deciding whether to materialize a transaction at all. It reports a transaction with no <code>amount</code> field and one with a duplicated <code>amount</code> field.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
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<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json_view.hpp></span>
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<span class=k>using</span><span class=w> </span><span class=n>json_document</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json_document</span><span class=p>;</span>
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<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
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<span class=p>{</span>
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<span class=w> </span><span class=c1>// validate that every transaction of a batch carries exactly one</span>
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<span class=w> </span><span class=c1>// "amount" field before materializing any of them into a nlohmann::json</span>
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<span class=w> </span><span class=c1>// value -- count() counts every member with the key</span>
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<span class=w> </span><span class=n>json_document</span><span class=w> </span><span class=n>batch</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json_document</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span>
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<span class=s> [</span>
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<span class=s> {"id": 1, "amount": 9.99},</span>
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<span class=s> {"id": 2},</span>
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<span class=s> {"id": 3, "amount": 5.00, "amount": 500.00}</span>
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<span class=s> ]</span>
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<span class=s> </span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
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<span class=w> </span><span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>transactions</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>batch</span><span class=p>.</span><span class=n>root</span><span class=p>();</span>
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<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=kt>size_t</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>0</span><span class=p>;</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o><</span><span class=w> </span><span class=n>transactions</span><span class=p>.</span><span class=n>size</span><span class=p>();</span><span class=w> </span><span class=o>++</span><span class=n>i</span><span class=p>)</span>
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<span class=w> </span><span class=p>{</span>
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<span class=w> </span><span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>transaction</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>transactions</span><span class=p>[</span><span class=n>i</span><span class=p>];</span>
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<span class=w> </span><span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>amounts</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>transaction</span><span class=p>.</span><span class=n>count</span><span class=p>(</span><span class=s>"amount"</span><span class=p>);</span>
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<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>amounts</span><span class=w> </span><span class=o>!=</span><span class=w> </span><span class=mi>1</span><span class=p>)</span>
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<span class=w> </span><span class=p>{</span>
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<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"transaction "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>" has "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>amounts</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>" </span><span class=se>\"</span><span class=s>amount</span><span class=se>\"</span><span class=s> fields</span><span class=se>\n</span><span class=s>"</span><span class=p>;</span>
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<span class=w> </span><span class=k>continue</span><span class=p>;</span>
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<span class=w> </span><span class=p>}</span>
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<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"transaction "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>": "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>transaction</span><span class=p>[</span><span class=s>"amount"</span><span class=p>].</span><span class=n>materialize</span><span class=p>().</span><span class=n>dump</span><span class=p>()</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
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<span class=w> </span><span class=p>}</span>
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<span class=p>}</span>
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</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=kc>transa</span><span class=err>c</span><span class=kc>t</span><span class=err>io</span><span class=kc>n</span><span class=w> </span><span class=mi>0</span><span class=p>:</span><span class=w> </span><span class=mf>9.99</span>
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<span class=kc>transa</span><span class=err>c</span><span class=kc>t</span><span class=err>io</span><span class=kc>n</span><span class=w> </span><span class=mi>1</span><span class=w> </span><span class=err>has</span><span class=w> </span><span class=mi>0</span><span class=w> </span><span class=s2>"amount"</span><span class=w> </span><span class=kc>f</span><span class=err>ields</span>
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<span class=kc>transa</span><span class=err>c</span><span class=kc>t</span><span class=err>io</span><span class=kc>n</span><span class=w> </span><span class=mi>2</span><span class=w> </span><span class=err>has</span><span class=w> </span><span class=mi>2</span><span class=w> </span><span class=s2>"amount"</span><span class=w> </span><span class=kc>f</span><span class=err>ields</span>
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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../find/ >find</a> - find a value in an object</li> <li><a href=../contains/ >contains</a> - checks whether a key exists</li> <li><a href=../../basic_json/count/ ><code>BasicJsonType::count</code></a> - the corresponding function of <code>basic_json</code></li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">¶</a></h2> <ul> <li>Added in version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>.</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 21, 2026 08:21:28 UTC">October 21, 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, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M416 32H31.9C14.3 32 0 46.5 0 64.3v383.4C0 465.5 14.3 480 31.9 480H416c17.6 0 32-14.5 32-32.3V64.3c0-17.8-14.4-32.3-32-32.3M135.4 416H69V202.2h66.5V416zM102.2 96a38.5 38.5 0 1 1 0 77 38.5 38.5 0 1 1 0-77m282.1 320h-66.4V312c0-24.8-.5-56.7-34.5-56.7-34.6 0-39.9 27-39.9 54.9V416h-66.4V202.2h63.7v29.2h.9c8.9-16.8 30.6-34.5 62.9-34.5 67.2 0 79.7 44.3 79.7 101.9z"/></svg> </a> <a href="https://www.xing.com/profile/Niels_LLine truncated
|
||||
@@ -0,0 +1,89 @@
|
||||
# nlohmann::basic_json_view::count
|
||||
|
||||
```
|
||||
size_type count(string_view_t key) const;
|
||||
size_type count(const char* key) const;
|
||||
size_type count(const string_t& key) const;
|
||||
```
|
||||
|
||||
Returns the number of members with key `key` if the value is an object, `0` otherwise. Members with a repeated key are all counted.
|
||||
|
||||
## Parameters
|
||||
|
||||
`key` (in) : key value of the element to count
|
||||
|
||||
## Return value
|
||||
|
||||
the number of members of the object with key `key` (duplicates included), or `0` if the value is not an object or has no such member.
|
||||
|
||||
## Exception safety
|
||||
|
||||
No-throw guarantee: this function never throws exceptions.
|
||||
|
||||
## Complexity
|
||||
|
||||
Linear in the number of members: unlike the lookup functions, `count()` does not stop at the first match, because it has to count every member with the key. Members are compared one after another, in document order. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content. The hash index of the keys of objects with 128 or more members does not help here, since every member is visited.
|
||||
|
||||
## Notes
|
||||
|
||||
This method always returns `0` when the value is not an object -- including a [discarded](https://json.nlohmann.me/api/basic_json_view/is_discarded/index.md) view.
|
||||
|
||||
If the source text repeats a key, `count()` returns more than `1` for it, even though the [materialized](https://json.nlohmann.me/api/basic_json_view/materialize/index.md) value (like the result of `parse()`) would have only one member with that key. Hence `count(key) > 1` detects duplicate keys. This is unlike the lookup functions ([`operator[]`](https://json.nlohmann.me/api/basic_json_view/operator%5B%5D/index.md), [`at`](https://json.nlohmann.me/api/basic_json_view/at/index.md), [`find`](https://json.nlohmann.me/api/basic_json_view/find/index.md), [`contains`](https://json.nlohmann.me/api/basic_json_view/contains/index.md)), which resolve to the *first* member; see the [Notes on duplicate keys](https://json.nlohmann.me/api/basic_json_view/operator%5B%5D/#notes) of `operator[]`.
|
||||
|
||||
## Examples
|
||||
|
||||
Example
|
||||
|
||||
The example below validates that every transaction of a batch carries exactly one `amount` field, using `count()` before deciding whether to materialize a transaction at all. It reports a transaction with no `amount` field and one with a duplicated `amount` field.
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <nlohmann/json_view.hpp>
|
||||
|
||||
using json_document = nlohmann::json_document;
|
||||
|
||||
int main()
|
||||
{
|
||||
// validate that every transaction of a batch carries exactly one
|
||||
// "amount" field before materializing any of them into a nlohmann::json
|
||||
// value -- count() counts every member with the key
|
||||
json_document batch = json_document::parse(R"(
|
||||
[
|
||||
{"id": 1, "amount": 9.99},
|
||||
{"id": 2},
|
||||
{"id": 3, "amount": 5.00, "amount": 500.00}
|
||||
]
|
||||
)");
|
||||
|
||||
const auto transactions = batch.root();
|
||||
for (std::size_t i = 0; i < transactions.size(); ++i)
|
||||
{
|
||||
const auto transaction = transactions[i];
|
||||
const auto amounts = transaction.count("amount");
|
||||
if (amounts != 1)
|
||||
{
|
||||
std::cout << "transaction " << i << " has " << amounts << " \"amount\" fields\n";
|
||||
continue;
|
||||
}
|
||||
std::cout << "transaction " << i << ": " << transaction["amount"].materialize().dump() << '\n';
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
transaction 0: 9.99
|
||||
transaction 1 has 0 "amount" fields
|
||||
transaction 2 has 2 "amount" fields
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [find](https://json.nlohmann.me/api/basic_json_view/find/index.md) - find a value in an object
|
||||
- [contains](https://json.nlohmann.me/api/basic_json_view/contains/index.md) - checks whether a key exists
|
||||
- [`BasicJsonType::count`](https://json.nlohmann.me/api/basic_json/count/index.md) - the corresponding function of `basic_json`
|
||||
|
||||
## Version history
|
||||
|
||||
- Added in version 3.13.0 unreleased.
|
||||
Reference in new issue
Block a user