NAME

std::variant::emplace - std::variant::emplace

Synopsis

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

constexpr std::variant_alternative_t<I, (since
variant>& C++20)

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

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....

* This overload participates in overload resolution only if
std::is_constructible_v<T, std::initializer_list<U>&, Args...> is true, and T
occurs exactly once 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.

* This overload participates in overload resolution only if
std::is_constructible_v<T_I, std::initializer_list<U>&, Args...> is true.
* It is a compile-time error if I is not less than sizeof...(Types).

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.

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 made constexpr
operations can be constexpr in C++20

See also

operator= assigns a variant
(public member function)