json/docs/mkdocs/docs/api/operator_ltlt.md

nlohmann::operator<<(basic_json), nlohmann::operator<<(json_pointer)

std::ostream& operator<<(std::ostream& o, const basic_json& j);      // (1)

std::ostream& operator<<(std::ostream& o, const json_pointer& ptr);  // (2)

1. Serialize the given JSON value j to the output stream o. The JSON value will be serialized using the dump member function.
   • The indentation of the output can be controlled with the member variable width of the output stream o. For instance, using the manipulator std::setw(4) on o sets the indentation level to 4 and the serialization result is the same as calling dump(4).
   • The indentation character can be controlled with the member variable fill of the output stream o. For instance, the manipulator std::setfill('\\t') sets indentation to use a tab character rather than the default space character.
2. Write a string representation of the given JSON pointer ptr to the output stream o. The string representation is obtained using the to_string member function.

Parameters

o (in, out)

stream to write to

j (in)

JSON value to serialize

ptr (in)

JSON pointer to write

Return value

the stream o

Exceptions

1. Throws type_error.316 if a string stored inside the JSON value is not UTF-8 encoded. Note that unlike the dump member functions, no error_handler can be set.
2. None.

Complexity

Linear.

Notes

!!! warning "Deprecation"

Function  `#!cpp std::ostream& operator<<(std::ostream& o, const basic_json& j)` replaces function
`#!cpp std::ostream& operator>>(const basic_json& j, std::ostream& o)` which has been deprecated in version 3.0.0.
It will be removed in version 4.0.0. Please replace calls like `#!cpp j >> o;` with `#!cpp o << j;`.

Examples

??? example "Example: (1) serialize JSON value to stream"

The example below shows the serialization with different parameters to `width` to adjust the indentation level.

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

Output:

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

??? example "Example: (2) write JSON pointer to stream"

The example below shows how to write a JSON pointer to a stream.

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

Output:

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

Version history

1. Added in version 1.0.0. Added support for indentation character and deprecated #!cpp std::ostream& operator>>(const basic_json& j, std::ostream& o) in version 3.0.0.
2. Added in version 3.11.0.