Equality Operator Overloading in Allegro

问题

One of my friends is trying to overload an equality operator for comparing colours in Allegro, however it does not work,He gets the error "no match for operator==" This is overloaded outside the Color class/struct, the overloaded operator function is shown below:

typedef ALLEGRO_COLOR Color;
bool operator==(const Color& rhs) const
{
 if(_col.a==rhs.a && _col.b==rhs.b && _col.g==rhs.g && _col.r==rhs.r)
 return true;
 else
 return false;
}
.
.
.
//Data member
Color _col

Im thinking this does not work because the operator is defined & implemented outside the ALLEGRO_COLOR in Allegro,right? How can this problem be solved, is it possible to overload outside the Allegro Color struct.

回答1:

operator== is a binary operator, but you only have one parameter. Try this:

bool operator==(const Color& _col, const Color& rhs) { ... }

---

Postscript: code of this form:

if ( condition )
    return true;
else
    return false;

is needlessly verbose in C++. Better to do this:

return condition;

In your case, I'd prefer to see:

return _col.a==rhs.a && _col.b==rhs.b && _col.g==rhs.g && _col.r==rhs.r;

来源：https://stackoverflow.com/questions/7406284/equality-operator-overloading-in-allegro