The semantics

Objects of type optional<T> are intended to be used in places where objects of type T would but which might be uninitialized. Hence, optional<T>'s purpose is to formalize the additional possibly uninitialized state. From the perspective of this role, optional<T> can have the same operational semantics of T plus the additional semantics corresponding to this special state. As such, optional<T> could be thought of as a supertype of T. Of course, we can't do that in C++, so we need to compose the desired semantics using a different mechanism. Doing it the other way around, that is, making optional<T> a subtype of T is not only conceptually wrong but also impractical: it is not allowed to derive from a non-class type, such as a built-in type.

We can draw from the purpose of optional<T> the required basic semantics:

Default Construction: To introduce a formally uninitialized wrapped object.
Direct Value Construction via copy: To introduce a formally initialized wrapped object whose value is obtained as a copy of some object.
Deep Copy Construction: To obtain a new yet equivalent wrapped object.
Direct Value Assignment (upon initialized): To assign a value to the wrapped object.
Direct Value Assignment (upon uninitialized): To initialize the wrapped object with a value obtained as a copy of some object.
Assignment (upon initialized): To assign to the wrapped object the value of another wrapped object.
Assignment (upon uninitialized): To initialize the wrapped object with value of another wrapped object.
Deep Relational Operations (when supported by the type T): To compare wrapped object values taking into account the presence of uninitialized states.
Value access: To unwrap the wrapped object.
Initialization state query: To determine if the object is formally initialized or not.
Swap: To exchange wrapped objects. (with whatever exception safety guarantees are provided by T's swap).
De-initialization: To release the wrapped object (if any) and leave the wrapper in the uninitialized state.

Additional operations are useful, such as converting constructors and converting assignments, in-place construction and assignment, and safe value access via a pointer to the wrapped object or null.

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The semantics