How to overload opAssign operator "globally" in C++

Question

Just curious about how to overload them.

The opAssign operators are like addAssign(+=) and subAssign(-=).

"globally" means they are not overloaded as member functions, but just a operator act on operands

For these opAssign operators, they are binary operators.(they receive two operands) Therefore two parameters are needed.

I found no examples on the web.....

score 9 · Accepted Answer · edited Oct 04 '09 at 16:24

9

Here's a trivial example of defining operator+=:

struct Foo{
    int x;
};

Foo& operator+=(Foo& lhs, const Foo& rhs) {
    lhs.x += rhs.x;
    return lhs;
}

edited Oct 04 '09 at 16:24

Magnus Hoff

21,529
9
63
82

answered Oct 04 '09 at 15:49

sepp2k

363,768
54
674
675

score 3 · Answer 2 · edited Oct 05 '09 at 10:03

3

The assignment operator (=) is special in that it always needs to be a non-static member function as per "§13.5.3 Assignment" of the C++ standard.

An assignment operator shall be implemented by a non-static member function with exactly one parameter

The same is true for the function call operator and the subscript operator. Other "assignment" operators (+=, -=, *=, etc) can be free binary functions.

edited Oct 05 '09 at 10:03

Magnus Hoff

21,529
9
63
82

answered Oct 04 '09 at 16:38

sellibitze

27,611
3
75
95

ASFAIU Bossliaw was asking about the operators that combine assignment with some other operation (`+=` etc.), not about pure assignment (`=`). – sbi Oct 04 '09 at 22:17
Tried to realize this: The Microsoft-Compiler seems to ignore the ´non-static member function´ overloading the '='-operator. Why is this? – Sam Ginrich Apr 06 '22 at 12:56

How to overload opAssign operator "globally" in C++

2 Answers2