What is use of the ref-qualifier `const &&`?

Question

I\'ve been digging around ref-qualifiers a bit, following on a previous question.

Given the code sample below;

#include 
#include <         


        

Answer 1

Suppose we have a type with a mutable state. Then const&& will both allow us to mutate that state, and indicate that such mutation is safe.

struct bar;

struct foo {
  mutable std::vector state;
  operator bar() const&;
  operator bar() const&&;
};

const is not absolute.

Barring mutable state, it is not safe to cast away const in a const&& method in order to extract state, because extracting state in this way from an actual const object is undefined behavior.