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std::optional::emplace

template< class... Args > T& emplace( Args&&... args );		(since C++17)
template< class U, class... Args > T& emplace( std::initializer_list<U> ilist, Args&&... args );		(since C++17)

Constructs the contained value in-place. If *this already contains a value before the call, the contained value is destroyed by calling its destructor.

1) Initializes the contained value by direct-initializing (but not direct-list-initializing) with std::forward<Args>(args)... as parameters.

2) Initializes the contained value by calling its constructor with ilist, std::forward<Args>(args)... as parameters. This overload only participates in overload resolution if std::is_constructible<T, std::initializer_list<U>&, Args&&...>::value is true.

Parameters

args...	-	the arguments to pass to the constructor
ilist	-	the initializer list to pass to the constructor
Type requirements
-`T` must be constructible from `Args...`
-`T` must be constructible from `std::initializer_list` and `Args...`

Return value

A reference to the new contained value.

Exceptions

Any exception thrown by the selected constructor of T. If an exception is thrown, *this does not contain a value after this call (the previously contained value, if any, had been destroyed).

std::optional::emplace

Parameters

Return value

Exceptions

See also