Any instance of
optional<T> at any given time either contains a value or does not contain a value
.When an instance of
optional<T> contains a value,
it means that an object of type
T, referred to as the optional object's
contained value,
is allocated within the storage of the optional object
.Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained value
.When an object of type
optional<T> is contextually converted to
bool,
the conversion returns
true if the object contains a value;
otherwise the conversion returns
false.