ref-qualifier

https://en.cppreference.com/w/cpp/language/member_functions

Member functions with ref-qualifier

An implicit object member function can be declared with no ref-qualifier, with an lvalue ref-qualifier (the token & after the parameter list) or the rvalue ref-qualifier (the token && after the parameter list). During overload resolution, an implicit object member function with a cv-qualifier sequence of class X is treated as follows:

• no ref-qualifier: the implicit object parameter has type lvalue reference to cv-qualified X and is additionally allowed to bind rvalue implied object argument
• lvalue ref-qualifier: the implicit object parameter has type lvalue reference to cv-qualified X
• rvalue ref-qualifier: the implicit object parameter has 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"
}

从示例中可以看出，成员函数用右值限定符修饰后只能通过右值对象调用，而左值限定符修饰的成员函数则可以通过左值对象和右值对象调用。