Meaning of ampersand in method declaration

Question

I know what a const qualifier means in a method declaration (making *this const), but I can't fathom what the ampersands in these lines means:

MyClass &operator =(const MyClass&) & = default;
//                                  ^- this one
bool operator ==(const MyClass &right) const &;
//                                 that one -^

Isn't decltype(*this) always MyClass& / const MyClass&? So what does the ampersand mean in here?

Answer 1

1. So what does the ampersand mean in here?

It means ref-qualified member functions:

A non-static member function can be declared with either an lvalue ref-qualifier (the token & after the function name) or rvalue ref-qualifier (the token && after the function name). 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.

You can define both (lvalue/rvalue ref-qualifier), and the appropriate one will be picked up by overload resolution. Such as:

bool operator ==(const MyClass &right) const &;
bool operator ==(const MyClass &right) const &&;

2. Isn't decltype(*this) always MyClass& / const MyClass&?

Note the type of *this won't change even in the rvalue ref-qualified function.

Note: unlike cv-qualification, ref-qualification does not change the properties of the this pointer: within a rvalue ref-qualified function, *this remains an lvalue expression.