std::variant<Types...>::emplace - cppreference.com (original) (raw)

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

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....

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....

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).

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

A reference to the new contained value.

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

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

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

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