std::variant<Types...>::emplace

Creates a new value in-place, in an existing variant object

# Declarations

template< class T, class... Args >
T& emplace( Args&&... args );

(since C++17) (constexpr since C++20)

template< class T, class U, class... Args >
T& emplace( std::initializer_list<U> il, Args&&... args );

(since C++17) (constexpr since C++20)

template< std::size_t I, class... Args >
std::variant_alternative_t<I, variant>& emplace( Args&&... args );

(since C++17) (constexpr since C++20)

template< std::size_t I, class U, class... Args >
std::variant_alternative_t<I, variant>&
emplace( std::initializer_list<U> il, Args&&... args );

(since C++17) (constexpr since C++20)

# Parameters

args: constructor arguments to use when constructing the new value
il: initializer_list argument to use when constructing the new value

# Return value

A reference to the new contained value.

# Notes

Feature-test macro Value Std Feature __cpp_lib_variant 202106L (C++20)(DR) Fully constexpr std::variant (1-4)

# Example

#include <iostream>
#include <string>
#include <variant>
 
int main()
{
    std::variant<std::string> v1;
    v1.emplace<0>("abc"); // OK
    std::cout << std::get<0>(v1) << '\n';
    v1.emplace<std::string>("def"); // OK
    std::cout << std::get<0>(v1) << '\n';
 
    std::variant<std::string, std::string> v2;
    v2.emplace<1>("ghi"); // OK
    std::cout << std::get<1>(v2) << '\n';
    // v2.emplace<std::string>("abc"); -> Error
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
P2231R1	C++20	emplace was not constexpr while the required operations can be constexpr in C++20	made constexpr

# See also

operator=