Question

How do I use the following struct:

struct point 
{
    int x;
    int y;
    int z;
};

as a key for std::map<point, bool>? How should I define operator< for two points?

Answer 1

Standard library containers like std::map require that your ordering is a “Strict Weak Ordering”, so you have to be very careful when designing one.

The typical approach for a 3-ary tuple looks like this:

bool operator<(const point& other) const
{
   if (x != other.x)
       return (x < other.x);

   if (y != other.y)
       return (y < other.y);

   return (z < other.z);
}

It’s like a comparator for just x, but with the difference that if the two xs are the same, you fall through to compare ys. If they are the same, then similarly you fall through to the z comparison.