What is the purpose of Ref-qualified member functions ?

Question

While reading http://en.cppreference.com/w/cpp/language/member_functions, I came across something, I haven't seen before: lvalue/rvalue Ref-qualified member functions. What would be their purpose ?

Answer 1

Just read down below:

During overload resolution, non-static cv-qualified member function of class X is treated as a function that takes an implicit parameter of type lvalue reference to cv-qualified X if it has no ref-qualifiers or if it has the lvalue ref-qualifier. Otherwise (if it has rvalue ref-qualifier), it is treated as a function taking an implicit parameter of type rvalue reference to cv-qualified X.

Example

#include <iostream>
struct S {
    void f() & { std::cout << "lvalue\n"; }
    void f() &&{ std::cout << "rvalue\n"; }
};
 
int main(){
    S s;
    s.f();            // prints "lvalue"
    std::move(s).f(); // prints "rvalue"
    S().f();          // prints "rvalue"
}

So during overload resolution the compiler looks for the function &-qualified if the caller object is an lvalue or for the function &&-qualified if the caller object is an rvalue.