rvalue ref-qualifiers for STL containers

Question

Why element access member functions of STL containers, e.g. std::array::operator[] or std::vector::operator[] do not have rvalue ref-qualifier over

Answer 1

There isn't a particular problem with adding ref-qualifiers to everything that returns references, however that basically doubles the number of members, which will generally have identical implementations apart from wrapping the return in std::move.

class A
{
    int a;

    int& operator[](std::size_t pos) & { return a; }
    int&& operator[](std::size_t pos) && { return std::move(a); }
};

The standard library has declined to provide these overloads, in the same way that it has declined to provide many volatile overloads. In this case, you can just std::move the & value, where you need it.

If you are writing your own containers, then there is no safeness reason to avoid such overloads. It does increase the maintenance burden, so I would advise against it.