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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?
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orx is an optional reference. It means that it either represents a reference (bound to another object, like all references), or an empty optional (so bound to nothing).
The class template std::optional manages an optional contained value, i.e. a value that may or may not be present. A common use case for optional is the return value of a function that may fail.
std::reference_wrapper is a class template that wraps a reference in a copyable, assignable object. It is frequently used as a mechanism to store references inside standard containers (like std::vector) which cannot normally hold references.
What's more, std::optional doesn't need to allocate any memory on the free store. std::optional is a part of C++ vocabulary types along with std::any , std::variant and std::string_view .
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.
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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:
std::optional.-> 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.
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