Question

I’m trying to use a simple structure as a map key:

class Foo{
 .
 .
 .
  struct index{
    int x;
    int y;
    int z;
  };

  bool operator<(const index a, const index b);
  .
  .
  .
}

And the function itslef:

bool Foo::operator<(const index a, const index b){
  bool out = True;
  if (a.x == b.x){
    if (a.y == b.y){
      if (a.z >= b.z) out = false;
    }
    else if(a.y > b.y) out = false;
  } else if (a.x > b.x) out = false;
  return out;
}

However, when I compile I get an error:

memMC.h:35: error: ‘bool Foo::operator<(Foo::index,
Foo::index)’ must take exactly one argument

As I understand this, the compilers wants to compare index to this Foo. How can I overload the operator then?

Answer 1

If you want to compare two indexes, move the overload inside the index structure:

struct index{
    int x;
    int y;
    int z;
    bool operator<(const index& a) const;
};

If you want to compare a Foo and an index (I doubt that, but I’ll just put this here just in case), remove the second parameter, as it’s not needed:

class Foo{
  //...
  bool operator<(const index& a) const;
};

Note that you should pass the index parameter by reference, to prevent unnecesarry copying.

EDIT: As Als correctly pointed out, this operator should be const.