std::optional specialization for reference types

问题:

Why std::optional (std::experimental::optional in libc++ at the moment) does not have specialization for reference types (compared with boost::optional)?

I think it would be very useful option.

Is there some object with reference to maybe already existing object semantics in STL?

回答1:

When n3406 (revision #2 of the proposal) was discussed, some committee members were uncomfortable with optional references. In n3527 (revision #3), the authors decided to make optional references an auxiliary proposal, to increase the chances of getting optional values approved and put into what became C++14. While optional didn't quite make it into C++14 for various other reasons, the committee did not reject optional references and is free to add optional references in the future should someone propose it.

回答2:

There is indeed something that has reference to maybe existing object semantics. It is called a (const) pointer. A plain old non-owning pointer. There are three differences between references and pointers:

1. Pointers can be null, references can not. This is exactly the difference you want to circumvent with std::optional.
2. Pointers can be redirected to point to something else. Make it const, and that difference disappears as well.
3. References need not be dereferenced by -> or *. This is pure syntactic sugar and possible because of 1. And the pointer syntax (dereferencing and convertible to bool) is exactly what std::optional provides for accessing the value and testing its presence.

Update: optional is a container for values. Like other containers (vector, for example) it is not designed to contain references. If you want an optional reference, use a pointer, or if you indeed need an interface with a similar syntax to std::optional, create a small (and trivial) wrapper for pointers.

Update2: As for the question why there is no such specialization: because the committee simply did opt it out. The rationale might be found somewhere in the papers. It possibly is because they considered pointers to be sufficient.

回答3:

If I would hazard a guess, it would be because of this sentence in the specification of std::experimental::optional. (Section 5.2, p1)

A program that necessitates the instantiation of template optional for a reference type, or for possibly cv-qualified types in_place_t or nullopt_t is ill-formed.