std::optional::emplace (3) Linux Manual Page

std::optional<T>::emplace – std::optional<T>::emplace

Synopsis

template< class... Args >                                      (since C++17)

T& emplace( Args&&... args );

template< class U, class... Args >                             (since C++17)

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

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… for overload (1)
–
T must be constructible from std::initializer_list and Args… for overload (2)

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).

See also

assigns contents

operator= (public member function)