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

Question

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.

See also the following answer for the usage with the assignment operator: http://stackoverflow.com/questions/13099942/should-i-still-return-const-objects-in-c11/13099997#13099997 — nosid, Jun 30 '14 at 06:54

score 2 · Accepted Answer · answered Jun 30 '14 at 06:55

2

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.

answered Jun 30 '14 at 06:55

mpark

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So without the `&` you could assign to rvalues of the class? – Praxeolitic Jun 30 '14 at 06:57

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

1 Answers1

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