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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

Deprecation

Function std::ostream& operator<<(std::ostream& o, const basic_json& j) replaces function 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 j >> o; with o << j;.

Examples

Example: (1) serialize JSON value to stream

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

#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // create JSON values
    json j_object = {{"one", 1}, {"two", 2}};
    json j_array = {1, 2, 4, 8, 16};

    // serialize without indentation
    std::cout << j_object << "\n\n";
    std::cout << j_array << "\n\n";

    // serialize with indentation
    std::cout << std::setw(4) << j_object << "\n\n";
    std::cout << std::setw(2) << j_array << "\n\n";
    std::cout << std::setw(1) << std::setfill('\t') << j_object << "\n\n";
}

Output:

{"one":1,"two":2}

[1,2,4,8,16]

{
    "one": 1,
    "two": 2
}

[
  1,
  2,
  4,
  8,
  16
]

{
    "one": 1,
    "two": 2
}
Example: (2) write JSON pointer to stream

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

#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // create JSON poiner
    json::json_pointer ptr("/foo/bar/baz");

    // write string representation to stream
    std::cout << ptr << std::endl;
}

Output:

/foo/bar/baz

Version history

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

Last update: September 18, 2022