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<span class=n>json</span><span class=w> </span><span class=n>a</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=mi>3</span><span class=p>,</span><span class=w> </span><span class=mi>4</span><span class=p>};</span>
<span class=n>a</span><span class=p>.</span><span class=n>erase</span><span class=p>(</span><span class=mi>1</span><span class=p>);</span><span class=w> </span><span class=c1>// [1,3,4] (erase by index)</span>
</code></pre></div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/push_back/ ><code>push_back</code></a> / <a href=../../api/basic_json/emplace_back/ ><code>emplace_back</code></a> - append to an array</li> <li><a href=../../api/basic_json/emplace/ ><code>emplace</code></a> - insert into an object if the key is absent</li> <li><a href=../../api/basic_json/update/ ><code>update</code></a> - merge objects</li> <li><a href=../../api/basic_json/erase/ ><code>erase</code></a> / <a href=../../api/basic_json/clear/ ><code>clear</code></a> - remove elements</li> <li><a href=../json_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch and Diff</a> and <a href=../merge_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch</a> - structured modifications</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="July 4, 2026 09:13:25 UTC">July 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> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/push_back/ ><code>push_back</code></a> / <a href=../../api/basic_json/emplace_back/ ><code>emplace_back</code></a> - append to an array</li> <li><a href=../../api/basic_json/emplace/ ><code>emplace</code></a> - insert into an object if the key is absent</li> <li><a href=../../api/basic_json/update/ ><code>update</code></a> - merge objects</li> <li><a href=../../api/basic_json/erase/ ><code>erase</code></a> / <a href=../../api/basic_json/clear/ ><code>clear</code></a> - remove elements</li> <li><a href=../json_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch and Diff</a> and <a href=../merge_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch</a> - structured modifications</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="July 8, 2026 18:18:24 UTC">July 8, 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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features/modifying_values/index.md
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# Modifying values
Once a JSON value exists, its content can be changed: elements can be added, replaced, merged, and removed. This page gives an overview of the available operations. For read access, see [element access](https://json.nlohmann.me/features/element_access/index.md).
## Adding to arrays
New elements are appended to an array with [`push_back`](https://json.nlohmann.me/api/basic_json/push_back/index.md) or constructed in place with [`emplace_back`](https://json.nlohmann.me/api/basic_json/emplace_back/index.md). If the value is `null`, it is converted to an array first, so these functions can also be used to build an array from scratch.
```
json j; // null
j.push_back(1); // [1]
j.push_back(2); // [1,2]
j.emplace_back(3); // [1,2,3]
// operator+= is a shorthand for push_back
j += 4; // [1,2,3,4]
```
## Adding to objects
The most common way to add or replace a member is [`operator[]`](https://json.nlohmann.me/features/element_access/unchecked_access/index.md), which inserts the key if it does not exist yet:
```
json j;
j["name"] = "Mary"; // {"name":"Mary"}
j["name"] = "John"; // {"name":"John"} (replaced)
```
[`emplace`](https://json.nlohmann.me/api/basic_json/emplace/index.md) inserts a member only if the key is not already present, and reports whether the insertion happened — useful for "add if absent" semantics.
## Merging objects
To merge one object into another, [`update`](https://json.nlohmann.me/api/basic_json/update/index.md) copies all members from another object, overwriting existing keys (similar to Python's `dict.update`). This is the idiomatic way to combine two objects.
Example
```
#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
using namespace nlohmann::literals;
int main()
{
// create two JSON objects
json o1 = R"( {"color": "red", "price": 17.99, "names": {"de": "Flugzeug"}} )"_json;
json o2 = R"( {"color": "blue", "speed": 100, "names": {"en": "plane"}} )"_json;
json o3 = o1;
// add all keys from o2 to o1 (updating "color", replacing "names")
o1.update(o2);
// add all keys from o2 to o1 (updating "color", merging "names")
o3.update(o2, true);
// output updated object o1 and o3
std::cout << std::setw(2) << o1 << '\n';
std::cout << std::setw(2) << o3 << '\n';
}
```
Output:
```
{
"color": "blue",
"names": {
"en": "plane"
},
"price": 17.99,
"speed": 100
}
{
"color": "blue",
"names": {
"de": "Flugzeug",
"en": "plane"
},
"price": 17.99,
"speed": 100
}
```
For a recursive merge that follows [RFC 7386](https://tools.ietf.org/html/rfc7386), see [JSON Merge Patch](https://json.nlohmann.me/features/merge_patch/index.md). To apply a sequence of well-defined edit operations, see [JSON Patch](https://json.nlohmann.me/features/json_patch/index.md).
## Removing elements
Elements are removed with [`erase`](https://json.nlohmann.me/api/basic_json/erase/index.md), which accepts an object key, an array index, or an iterator. [`clear`](https://json.nlohmann.me/api/basic_json/clear/index.md) empties a value while keeping its type, and [`operator[]`](https://json.nlohmann.me/features/element_access/unchecked_access/index.md) combined with assignment can overwrite a value entirely.
```
json j = {{"a", 1}, {"b", 2}, {"c", 3}};
j.erase("b"); // {"a":1,"c":3}
json a = {1, 2, 3, 4};
a.erase(1); // [1,3,4] (erase by index)
```
## See also
- [`push_back`](https://json.nlohmann.me/api/basic_json/push_back/index.md) / [`emplace_back`](https://json.nlohmann.me/api/basic_json/emplace_back/index.md) - append to an array
- [`emplace`](https://json.nlohmann.me/api/basic_json/emplace/index.md) - insert into an object if the key is absent
- [`update`](https://json.nlohmann.me/api/basic_json/update/index.md) - merge objects
- [`erase`](https://json.nlohmann.me/api/basic_json/erase/index.md) / [`clear`](https://json.nlohmann.me/api/basic_json/clear/index.md) - remove elements
- [JSON Patch and Diff](https://json.nlohmann.me/features/json_patch/index.md) and [JSON Merge Patch](https://json.nlohmann.me/features/merge_patch/index.md) - structured modifications