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

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

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
}

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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