Optional references

This library allows the template parameter T to be of reference type: T&, and to some extent, T const&.

However, since references are not real objects some restrictions apply and some operations are not available in this case:

Converting constructors
Converting assignment
InPlace construction
InPlace assignment
Value-access via pointer

Also, even though optional<T&> treats it wrapped pseudo-object much as a real value, a true real reference is stored so aliasing will ocurr:

Copies of optional<T&> will copy the references but all these references will nonetheless reefer to the same object.
Value-access will actually provide access to the referenced object rather than the reference itself.

Rvalue references and lvalue references to const have the ability in C++ to extend the life time of a temporary they bind to. Optional references do not have this capability, therefore to avoid surprising effects it is not possible to initialize an optional references from a temporary. Optional rvalue references are disabled altogether. Also, the initialization and assignment of an optional reference to const from rvalue reference is disabled.

const int& i = 1;            // legal
optional<const int&> oi = 1; // illegal