Operator overloading confusion

Question

I know there have been many questions posted about operator overload, but I can't get my code to work with any of the examples I find. Yes, I have taken a look at this link, but I am still having trouble understanding this.

I am trying to overload just the prefix operator, but I get an error saying that postfix expects an int. I am NOT trying to modify the postfix operator. I have a .h file, and a .cpp file. Inside of my .h file I have something like:

class X{
public:
        /* ... declarations of X ... */
        X& operator--(X& x);
};

And inside of my .cpp file I have something like:

#include "X.h"
namespace X{
X& X::operator--(X& x){/*...*/}
}

Can anyone tell me what I am doing incorrectly?

Can you show an example of trying to use your function and the exact compiler error. — Cory Kramer, Oct 04 '15 at 16:00
Personally I avoid operator overloading as it more trouble than it is worth — Ed Heal, Oct 04 '15 at 16:02
Why does the cpp file have class X's member functions in the X namespace? That's not the case in the header file. — Anon Mail, Oct 04 '15 at 16:03
Here's a good example http://www.learncpp.com/cpp-tutorial/97-overloading-the-increment-and-decrement-operators/ — ashiquzzaman33, Oct 04 '15 at 16:11

score 2 · Accepted Answer · answered Oct 04 '15 at 16:03

2

The prefix -- operator takes no arguments. Its definition should be as follows:

public:
    /* ... declarations of X ... */
    X& operator--();
};

And similarly when implementing it:

#include "X.h"
namespace X {
    X& X::operator--() {/*...*/}
}

answered Oct 04 '15 at 16:03

Mureinik

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OK, this makes more sense now. Thank you. – K. Shores Oct 04 '15 at 16:09

Christian Hackl · Answer 2 · 2015-10-04T16:08:59.193

X& operator--(X& x);

Must be:

X& operator--();

Background: The compiler sees that you've given your -- operator an argument. The postfix version of -- takes an argument, so it thinks you mean that one. But the type of that argument must by convention be int. So it tells you exactly that: postfix 'X& X::operator--(X&)' must take 'int' as its argument.

The compiler could also tell you something like prefix 'X& X::operator--(X&)' must not take any argument if it wanted to. But, oh well. That's the way it is. Producing any diagnostic message is enough to conform to the C++ standard.

Operator overloading confusion

2 Answers2