重载类中的<运算符

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 .

Answer 2

< is binary infix comparison operator ie it takes two arguments to compare to each other, So ideally it should be implemented as a free function, However if you implement it as a member function then it will take only one argument.

It shall compare the argument passed as parameter to the object on which the member function is being called.

Your member comparison operator should be like:

class Foo
{
    //...
    bool operator<(const index& rhs) const 
    { 
        /* comparison with *this */ 
    }
    //...
};