1) Constructs an empty object.

2,3) Copies(2) or moves(3) content ofother into a new instance, so that any content is equivalent in both type and value to those ofother prior to the constructor call, or empty ifother is empty.

4) Constructs an object with initial content an object of typestd::decay_t<ValueType>,direct-initialized fromstd::forward<ValueType>(value). Ifstd::is_copy_constructible<std::decay_t<ValueType>>::value isfalse, the program is ill-formed. This overload participates in overload resolution only ifstd::decay_t<ValueType> is not the same type asany.

2,4) Throws any exception thrown by the constructor of the contained type.