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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/ordered_map/" rel="canonical"><link href=../ordered_json/ rel=prev><link href=../macros/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>ordered_map - 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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<svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> ordered_map </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio 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15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 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<span class=w> </span><span class=k>class</span><span class=w> </span><span class=nc>Allocator</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>allocator</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>Key</span><span class=p>,</span><span class=w> </span><span class=n>T</span><span class=o>&gt;&gt;&gt;</span>
<span class=k>struct</span><span class=w> </span><span class=nc>ordered_map</span><span class=w> </span><span class=o>:</span><span class=w> </span><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=n>pair</span><span class=o>&lt;</span><span class=k>const</span><span class=w> </span><span class=n>Key</span><span class=p>,</span><span class=w> </span><span class=n>T</span><span class=o>&gt;</span><span class=p>,</span><span class=w> </span><span class=n>Allocator</span><span class=o>&gt;</span><span class=p>;</span>
</code></pre></div> <p>A minimal map-like container that preserves insertion order for use within <a href=../ordered_json/ ><code>nlohmann::ordered_json</code></a> (<code>nlohmann::basic_json&lt;ordered_map&gt;</code>).</p> <h2 id=template-parameters>Template parameters<a class=headerlink href=#template-parameters title="Permanent link">&para;</a></h2> <dl> <dt><code>Key</code></dt> <dd>key type</dd> <dt><code>T</code></dt> <dd>mapped type</dd> <dt><code>IgnoredLess</code></dt> <dd>comparison function (ignored and only added to ensure compatibility with <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>)</dd> <dt><code>Allocator</code></dt> <dd>allocator type</dd> </dl> <h2 id=iterator-invalidation>Iterator invalidation<a class=headerlink href=#iterator-invalidation title="Permanent link">&para;</a></h2> <p>The type uses a <code>std::vector</code> to store object elements. Therefore, adding elements can yield a reallocation in which case all iterators (including the <code>end()</code> iterator) and all references to the elements are invalidated.</p> <h2 id=member-types>Member types<a class=headerlink href=#member-types title="Permanent link">&para;</a></h2> <ul> <li><strong>key_type</strong> - key type (<code>Key</code>)</li> <li><strong>mapped_type</strong> - mapped type (<code>T</code>)</li> <li><strong>Container</strong> - base container type (<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=n>pair</span><span class=o>&lt;</span><span class=k>const</span><span class=w> </span><span class=n>Key</span><span class=p>,</span><span class=w> </span><span class=n>T</span><span class=o>&gt;</span><span class=p>,</span><span class=w> </span><span class=n>Allocator</span><span class=o>&gt;</span></code>)</li> <li><strong>iterator</strong></li> <li><strong>const_iterator</strong></li> <li><strong>size_type</strong></li> <li><strong>value_type</strong></li> <li><strong>key_compare</strong> - key comparison function <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>equal_to</span><span class=o>&lt;</span><span class=n>Key</span><span class=o>&gt;</span><span class=w> </span><span class=c1>// until C++14</span>
<span class=n>std</span><span class=o>::</span><span class=n>equal_to</span><span class=o>&lt;&gt;</span><span class=w> </span><span class=c1>// since C++14</span>
</code></pre></div></li> </ul> <h2 id=member-functions>Member functions<a class=headerlink href=#member-functions title="Permanent link">&para;</a></h2> <ul> <li>(constructor)</li> <li>(destructor)</li> <li><strong>emplace</strong></li> <li><strong>operator[]</strong></li> <li><strong>at</strong></li> <li><strong>erase</strong></li> <li><strong>count</strong></li> <li><strong>find</strong></li> <li><strong>insert</strong></li> </ul> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">&para;</a></h2> <details class=example> <summary>Example</summary> <p>The example shows the different behavior of <code>std::map</code> and <code>nlohmann::ordered_map</code>.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
</code></pre></div></li> </ul> <h2 id=member-functions>Member functions<a class=headerlink href=#member-functions title="Permanent link">&para;</a></h2> <ul> <li>(constructor)</li> <li>(destructor)</li> <li><strong>emplace</strong></li> <li><strong>operator[]</strong></li> <li><strong>at</strong></li> <li><strong>erase</strong></li> <li><strong>count</strong></li> <li><strong>find</strong></li> <li><strong>insert</strong></li> </ul> <h2 id=complexity>Complexity<a class=headerlink href=#complexity title="Permanent link">&para;</a></h2> <p>Because the elements are stored in a <code>std::vector</code> in insertion order, there is no index to look a key up by. Every key-based operation performs a <strong>linear scan</strong> over the stored elements. With <code>n</code> denoting the number of elements in the container:</p> <table> <thead> <tr> <th>Operation</th> <th>Complexity</th> <th>Note</th> </tr> </thead> <tbody> <tr> <td><strong>emplace</strong></td> <td>O(n)</td> <td>scans for an existing key, then appends (amortized O(1))</td> </tr> <tr> <td><strong>operator[]</strong></td> <td>O(n)</td> <td>delegates to <strong>emplace</strong> (non-const) or <strong>at</strong> (const)</td> </tr> <tr> <td><strong>at</strong></td> <td>O(n)</td> <td>throws <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>out_of_range</span></code> if the key is not found</td> </tr> <tr> <td><strong>find</strong></td> <td>O(n)</td> <td></td> </tr> <tr> <td><strong>count</strong></td> <td>O(n)</td> <td>the result is always 0 or 1</td> </tr> <tr> <td><strong>erase(key)</strong></td> <td>O(n)</td> <td>scan, then move the remaining elements one position down</td> </tr> <tr> <td><strong>erase(pos)</strong>, <strong>erase(first, last)</strong></td> <td>O(n)</td> <td>moves all elements after the erased range</td> </tr> <tr> <td><strong>insert(value)</strong></td> <td>O(n)</td> <td>equivalent to <strong>emplace</strong></td> </tr> <tr> <td><strong>insert(first, last)</strong></td> <td>O((n + m) * m)</td> <td>for <code>m</code> inserted elements</td> </tr> </tbody> </table> <p>This differs from <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>, where the same operations are O(log n).</p> <div class="admonition warning"> <p class=admonition-title>Quadratic cost of building large objects</p> <p>Because every insertion scans all elements inserted so far, building an object of <code>n</code> distinct keys costs <strong>O(n²)</strong> in total. This applies to filling an <a href=../ordered_json/ ><code>ordered_json</code></a> object key by key as well as to parsing one, since the parser inserts each key as it is read.</p> <p>The cost is negligible for the object sizes typically found in configuration files or <abbr title="Application Programming Interfaces">API</abbr> payloads, but it grows steeply for machine-generated objects with many thousands of keys. Measured with <code>-O2 -DNDEBUG</code> for parsing a flat object of <code>n</code> keys, relative to <code class=highlight><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span></code> (which uses <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>):</p> <table> <thead> <tr> <th><code>n</code></th> <th><code>json</code></th> <th><code>ordered_json</code></th> <th>factor</th> </tr> </thead> <tbody> <tr> <td>2000</td> <td>0.7 ms</td> <td>3.6 ms</td> <td>5×</td> </tr> <tr> <td>4000</td> <td>0.8 ms</td> <td>14.0 ms</td> <td>19×</td> </tr> <tr> <td>8000</td> <td>1.6 ms</td> <td>67.8 ms</td> <td>43×</td> </tr> <tr> <td>16 000</td> <td>3.3 ms</td> <td>181.6 ms</td> <td>54×</td> </tr> </tbody> </table> <p>If key order matters for objects of that size, consider a container with a lookup index, such as <a href="https://github.com/Tessil/ordered-map"><code>tsl::ordered_map</code></a> (<a href="https://github.com/nlohmann/json/issues/546#issuecomment-304447518">integration</a>), as the object type -- see <a href=../../features/object_order/ >object order</a>.</p> </div> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">&para;</a></h2> <details class=example> <summary>Example</summary> <p>The example shows the different behavior of <code>std::map</code> and <code>nlohmann::ordered_map</code>.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
<span class=c1>// simple output function</span>
@@ -51,4 +51,4 @@
<span class=err>m_s</span><span class=kc>t</span><span class=err>d</span><span class=w> </span><span class=err>=</span><span class=w> </span><span class=p>{</span><span class=w> </span><span class=err>o</span><span class=kc>ne</span><span class=p>:</span><span class=err>ei</span><span class=kc>ns</span><span class=w> </span><span class=kc>t</span><span class=err>hree</span><span class=p>:</span><span class=err>drei</span><span class=w> </span><span class=kc>t</span><span class=err>wo</span><span class=p>:</span><span class=err>zwei</span><span class=w> </span><span class=p>}</span>
<span class=err>m_ordered</span><span class=w> </span><span class=err>=</span><span class=w> </span><span class=p>{</span><span class=w> </span><span class=kc>t</span><span class=err>wo</span><span class=p>:</span><span class=err>zwei</span><span class=w> </span><span class=kc>t</span><span class=err>hree</span><span class=p>:</span><span class=err>drei</span><span class=w> </span><span class=err>o</span><span class=kc>ne</span><span class=p>:</span><span class=err>ei</span><span class=kc>ns</span><span class=w> </span><span class=p>}</span>
<span class=err>m_s</span><span class=kc>t</span><span class=err>d</span><span class=w> </span><span class=err>=</span><span class=w> </span><span class=p>{</span><span class=w> </span><span class=err>o</span><span class=kc>ne</span><span class=p>:</span><span class=err>ei</span><span class=kc>ns</span><span class=w> </span><span class=kc>t</span><span class=err>hree</span><span class=p>:</span><span class=err>drei</span><span class=w> </span><span class=kc>t</span><span class=err>wo</span><span class=p>:</span><span class=err>zwei</span><span class=w> </span><span class=p>}</span>
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ordered_json/ >ordered_json</a></li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">&para;</a></h2> <ul> <li>Added in version 3.9.0 to implement <a href=../ordered_json/ ><code>nlohmann::ordered_json</code></a>.</li> <li>Added <strong>key_compare</strong> member in version 3.11.0.</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="August 4, 2026 06:45:02 UTC">August 4, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ordered_json/ >ordered_json</a></li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">&para;</a></h2> <ul> <li>Added in version 3.9.0 to implement <a href=../ordered_json/ ><code>nlohmann::ordered_json</code></a>.</li> <li>Added <strong>key_compare</strong> member in version 3.11.0.</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="August 4, 2026 06:45:50 UTC">August 4, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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+33
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@@ -58,6 +58,39 @@ The type uses a `std::vector` to store object elements. Therefore, adding elemen
- **find**
- **insert**
## Complexity
Because the elements are stored in a `std::vector` in insertion order, there is no index to look a key up by. Every key-based operation performs a **linear scan** over the stored elements. With `n` denoting the number of elements in the container:
| Operation | Complexity | Note |
| -------------------------------------- | -------------- | -------------------------------------------------------- |
| **emplace** | O(n) | scans for an existing key, then appends (amortized O(1)) |
| **operator[]** | O(n) | delegates to **emplace** (non-const) or **at** (const) |
| **at** | O(n) | throws `std::out_of_range` if the key is not found |
| **find** | O(n) | |
| **count** | O(n) | the result is always 0 or 1 |
| **erase(key)** | O(n) | scan, then move the remaining elements one position down |
| **erase(pos)**, **erase(first, last)** | O(n) | moves all elements after the erased range |
| **insert(value)** | O(n) | equivalent to **emplace** |
| **insert(first, last)** | O((n + m) * m) | for `m` inserted elements |
This differs from `std::map`, where the same operations are O(log n).
Quadratic cost of building large objects
Because every insertion scans all elements inserted so far, building an object of `n` distinct keys costs **O(n²)** in total. This applies to filling an [`ordered_json`](https://json.nlohmann.me/api/ordered_json/index.md) object key by key as well as to parsing one, since the parser inserts each key as it is read.
The cost is negligible for the object sizes typically found in configuration files or API payloads, but it grows steeply for machine-generated objects with many thousands of keys. Measured with `-O2 -DNDEBUG` for parsing a flat object of `n` keys, relative to `nlohmann::json` (which uses `std::map`):
| `n` | `json` | `ordered_json` | factor |
| ------ | ------ | -------------- | ------ |
| 2000 | 0.7 ms | 3.6 ms | 5× |
| 4000 | 0.8 ms | 14.0 ms | 19× |
| 8000 | 1.6 ms | 67.8 ms | 43× |
| 16 000 | 3.3 ms | 181.6 ms | 54× |
If key order matters for objects of that size, consider a container with a lookup index, such as [`tsl::ordered_map`](https://github.com/Tessil/ordered-map) ([integration](https://github.com/nlohmann/json/issues/546#issuecomment-304447518)), as the object type -- see [object order](https://json.nlohmann.me/features/object_order/index.md).
## Examples
Example