C++ is it possible to overload the unary minus operator of an rvalue reference?

Question

is it possible to discern between these two methods? should one not mutate an rvalue when in this case seems perfectly reusable?

TYPE a;
TYPE b = -a;      // unary operator- of a TYPE& aka lvalue ref
TYPE c = -(a+b);  // unary operator- of a TYPE&& aka rvalue ref

Answer 1

You can use reference qualifier-s (or ref-qualifiers as in standard)

http://coliru.stacked-crooked.com/a/40905649dc0c14e7

example:

#include <iostream>
#include <string>
#include <vector>

class X
{
   public:
      int n;
      X(const X&) = default;
      X() : n(0) {}
      X(int n) : n(n) {}

      X operator- () const &  // lvalue ref-qualifier
      {
         std::cout << "&\n";
         X x(-n);
         return x;
      }

      X operator- () const && // rvalue ref-qualifier
      {
         std::cout << "&&\n";
         X x(-n);
         return x;
      }    

      friend X operator+(const X& lhs, const X& rhs) {
          return X(lhs.n + rhs.n);
      }
};

int main()
{
    X a;
    X b = -a;      // unary operator- of a TYPE& aka lvalue ref
    X c = -(a+b);
}

outputs:

&
&&