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

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std::variant
template < class T, class... Args >
T& emplace( Args&&... args ) ;
(1) (since C++17)
(constexpr since C++20)
template < class T, class U, class... Args >
T& emplace( std::initializer_list <U> il, Args&&... args ) ;
(2) (since C++17)
(constexpr since C++20)
template < std::size_t I, class... Args >
std::variant_alternative_t <I, variant> & emplace( Args&&... args ) ;
(3) (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 ) ;
(4) (since C++17)
(constexpr since C++20)

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

1) Equivalent to emplace<I> ( std::forward <Args> (args)...) , where I is the zero-based index of T in Types....
  • This overload participates in overload resolution only if std::is_constructible_v<T, Args...> is true, and T occurs exactly once in Types...
2) Equivalent to emplace<I> (il, std::forward <Args> (args)...) , where I is the zero-based index of T in Types....
3) First, destroys the currently contained value (if any). Then direct-initializes the contained value as if constructing a value of type T_I with the arguments std::forward <Args> (args) . If an exception is thrown, *this may become valueless_by_exception.
  • This overload participates in overload resolution only if std::is_constructible_v<T_I, Args...> is true
  • It is a compile-time error if I is not less than sizeof...(Types).
4) First, destroys the currently contained value (if any). Then direct-initializes the contained value as if constructing a value of type T_I with the arguments il, std::forward <Args> (args) . If an exception is thrown, *this may become valueless_by_exception.

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.

Exceptions

1-4) Any exception thrown during the initialization of the contained value.

Notes

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

Example

Run this code
#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
}

Output: 

abc
def
ghi

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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

 assigns a variant
(public member function)
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