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&&).

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

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¶

#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';
}

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

#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';
}

{"one":1,"two":2}
null
{"four":4,"one":1,"three":3,"two":2}
{"A":"a","B":"b"}

#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});
}

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

See also¶

• emplace_back add a value to an array
• operator+= add a value to an array/object

Version history¶

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