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Showing content from https://en.cppreference.com/w/cpp/language/../../cpp/utility/variant/emplace.html below:

std::variant<Types...>::emplace - cppreference.com

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

(1) (since C++17)
(constexpr since C++20) (2) (since C++17)
(constexpr since C++20) (3) (since C++17)
(constexpr since C++20) (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

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

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).

[edit] Parameters args - constructor arguments to use when constructing the new value il - initializer_list argument to use when constructing the new value [edit] Return value

A reference to the new contained value.

[edit] Exceptions

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

[edit] Notes [edit] Example [edit] 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 [edit] See also

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