C++ - On friend assignment-operator overloads

Question

I’ve been experimenting with creating code interfaces with C++ classes.

And a puzzling thing that came up in testing is overloading assignment operators. Take a look at the code below:

struct my_object {
  // This doesn't work. :(
  friend int& operator =(int& variable, my_object object)
  { variable = object.value; return variable; }

  // This works, but `int value; value <<= my_object{};` 
  // for `int value; value = my_object{};` doesn't seem clean...
  friend int& operator <<=(int& variable, my_object object)
  { variable = object.value; return variable; }
};

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My confusion is:
The first overload does not compile and logs this error to the console terminal.

'friend int& operator =(int&, my_object)' must be a nonstatic member function

But then, the second overload compiles with no errors (or warnings) whatsoever despite being near identical to the first one.

---

Why is the first operator overload invalid and the second one seemingly fine (at least by the GCC 6.3.0 compiler)?

Answer 1

The =, () (function call), and [] operators are required to be members of the class. Other operators generally are allowed to be either members or non-members.

It follows that the only time when you can overload the assignment operator is when defining the class on the left-hand side. There is no way to write a custom assignment operator whose left-hand argument is a non-class type.

In order to assign my_object to int, you need to write a conversion function instead:

struct my_object {
    operator int() const { return value; }
    // ...
    int value;
};