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nlohmann::basic_json::push_back

// (1)
void push_back(basic_json&& val);
void push_back(const basic_json& val);

// (2)
void push_back(const typename object_t::value_type& val);

// (3)
void push_back(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 push_back 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 push_back(const typename object_t::value_type&). Otherwise, init is converted to a JSON value and added using push_back(basic_json&&).

Parameters

val (in)
the value to add to the JSON array/object
init (in)
an initializer list

Exceptions

All functions can throw the following exception: - 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: (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.

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

using json = nlohmann::json;

int main()
{
    // create JSON values
    json array = {1, 2, 3, 4, 5};
    json null;

    // print values
    std::cout << array << '\n';
    std::cout << null << '\n';

    // add values
    array.push_back(6);
    array += 7;
    null += "first";
    null += "second";

    // print values
    std::cout << array << '\n';
    std::cout << null << '\n';
}

Output:

[1,2,3,4,5]
null
[1,2,3,4,5,6,7]
["first","second"]
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.

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

using json = nlohmann::json;

int main()
{
    // create JSON values
    json object = {{"one", 1}, {"two", 2}};
    json null;

    // print values
    std::cout << object << '\n';
    std::cout << null << '\n';

    // add values
    object.push_back(json::object_t::value_type("three", 3));
    object += json::object_t::value_type("four", 4);
    null += json::object_t::value_type("A", "a");
    null += json::object_t::value_type("B", "b");

    // print values
    std::cout << object << '\n';
    std::cout << null << '\n';
}

Output:

{"one":1,"two":2}
null
{"four":4,"one":1,"three":3,"two":2}
{"A":"a","B":"b"}
Example: (3) add to object from initializer list

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

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

using json = nlohmann::json;

int main()
{
    // create JSON values
    json object = {{"one", 1}, {"two", 2}};
    json null;

    // print values
    std::cout << object << '\n';
    std::cout << null << '\n';

    // add values:
    object.push_back({"three", 3});  // object is extended
    object += {"four", 4};           // object is extended
    null.push_back({"five", 5});     // null is converted to array

    // print values
    std::cout << object << '\n';
    std::cout << null << '\n';

    // would throw:
    //object.push_back({1, 2, 3});
}

Output:

{"one":1,"two":2}
null
{"four":4,"one":1,"three":3,"two":2}
[["five",5]]

Version history

  1. Since version 1.0.0.
  2. Since version 1.0.0.
  3. Since version 2.0.0.
Last update: May 17, 2022