json/docs/mkdocs/docs/api/basic_json/operator+=.md

nlohmann::basic_json::operator+=

// (1)
reference operator+=(basic_json&& val);
reference operator+=(const basic_json& val);

// (2)
reference operator+=(const typename object_t::value_type& val);

// (3)
reference operator+=(initializer_list_t init);

1. Appends the given element val to the end of the JSON array. If the function is called on a JSON null value, an empty array is created before appending val.

2. Inserts the given element val to the JSON object. If the function is called on a JSON null value, an empty object is created before inserting val.

3. This function allows using operator+= with an initializer list. In case

   1. the current value is an object,
   2. the initializer list init contains only two elements, and
   3. the first element of init is a string,

   init is converted into an object element and added using operator+=(const typename object_t::value_type&). Otherwise, init is converted to a JSON value and added using operator+=(basic_json&&).

Iterator invalidation

For all cases where an element is added to an array, a reallocation can happen, in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Otherwise, only the end() iterator is invalidated.

For ordered_json, also adding an element to an object can yield a reallocation which again invalidates all iterators and all references.

Parameters

val (in)

the value to add to the JSON array/object

init (in)

an initializer list

Return value

#!cpp *this

Exception safety

Strong guarantee: if an exception is thrown, there are no changes to any JSON value. As an exception, a #!json null value is converted to an empty array or object before the element is added and keeps that type if adding the element throws.

Exceptions

1. Throws type_error.308 when called on a type other than JSON array or null; example: "cannot use push_back() with number"
2. Throws type_error.308 when called on a type other than JSON object or null; example: "cannot use push_back() with number"
3. Throws type_error.308 when called on a type other than JSON array or null; example: "cannot use push_back() with number"

Complexity

1. Amortized constant.
2. Logarithmic in the size of the container, O(log(size())).
3. Linear in the size of the initializer list init.

Notes

(3) This function is required to resolve an ambiguous overload error, because pairs like {"key", "value"} can be both interpreted as object_t::value_type or std::initializer_list<basic_json>, see #235 for more information.

Examples

??? example "Example: (1) add element to array"

The example shows how `push_back()` and `+=` can be used to add elements to a JSON array. Note how the `null` value
was silently converted to a JSON array.

```cpp
--8<-- "examples/push_back.cpp"
```

Output:

```json
--8<-- "examples/push_back.output"
```

??? example "Example: (2) add element to object"

The example shows how `push_back()` and `+=` can be used to add elements to a JSON object. Note how the `null` value
was silently converted to a JSON object.

```cpp
--8<-- "examples/push_back__object_t__value.cpp"
```

Output:

```json
--8<-- "examples/push_back__object_t__value.output"
```

??? example "Example: (3) add to object from initializer list"

The example shows how initializer lists are treated as objects when possible.

```cpp
--8<-- "examples/push_back__initializer_list.cpp"
```

Output:

```json
--8<-- "examples/push_back__initializer_list.output"
```

See also

• emplace_back add a value to an array
• push_back add a value to an array/object

Version history

1. Added in version 1.0.0.
2. Added in version 1.0.0.
3. Added in version 2.0.0.