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

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 to use 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&&).

Parameters

val (in)

the value to add to the JSON array/object

init (in)

an initializer list

Return value

#!cpp *this

Exceptions

1. The function can throw the following exceptions:
   • Throws type_error.308 when called on a type other than JSON array or null; example: "cannot use operator+=() with number"
2. The function can throw the following exceptions:
   • Throws type_error.308 when called on a type other than JSON object or null; example: "cannot use operator+=() 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

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

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

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"
```

Version history

1. Since version 1.0.0.
2. Since version 1.0.0.
3. Since version 2.0.0.