What does & = in C& operator=(const C&) & = default; do? [duplicate]

问题

This question already has answers here:

What is “rvalue reference for *this”? (3 answers)

Closed 5 years ago.

A few questions on SO use a particular syntax for declaring default assignment operators.

Rule-of-Three becomes Rule-of-Five with C++11?

class C {
  C(const C&) = default;
  C(C&&) = default;
  C& operator=(const C&) & = default;
  C& operator=(C&&) & = default;
  virtual ~C() { }
};

I'm confused by the & = used for the assignment operators. After a quick test, default assignment operator declarations seem to compile and give the expected behavior with or without the additional ampersand.

I don't see the & = syntax on cppreference.

回答1:

The & there is a ref qualifier.

In that particular case it makes it so that the instance of C you want to assign to must be a non-const lvalue.

来源：https://stackoverflow.com/questions/24484365/what-does-in-c-operator-const-c-default-do