nlohmann::basic_json::emplace_back¶

template<class... Args>
reference emplace_back(Args&& ... args);

Creates a JSON value from the passed parameters args to the end of the JSON value. If the function is called on a JSON null value, an empty array is created before appending the value created from args.

Template parameters¶

Args: compatible types to create a basic_json object

Parameters¶

args (in): arguments to forward to a constructor of basic_json

Return value¶

reference to the inserted element

Exceptions¶

Throws type_error.311 when called on a type other than JSON array or null; example: "cannot use emplace_back() with number"

Complexity¶

Amortized constant.

Examples¶

Example

The example shows how emplace_back() 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.emplace_back(6);
    null.emplace_back("first");
    null.emplace_back(3, "second");

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

Output:

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

Version history¶

Since version 2.0.8.
Returns reference since 3.7.0.

Last update: May 1, 2022