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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

Iterator invalidation

By adding an element to the end of the 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.

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"]]

See also

Version history

  • Since version 2.0.8.
  • Returns reference since 3.7.0.