C++ std::set custom comparator

Question

Well, my problem is that I'm using an std::set with a custom comparator, something like:

class A
{
public:
    A(int x, int y):
        _x(x), _y(y)
    {
    }

    int hashCode(){ return (_y << 16) | _x; }

private:
    short int _y;
    short int _x;
};

struct comp
{
    bool operator() (A* g1, A* g2) const
    {
        return g1->hashCode() < g2->hashCode();
    }
};

So, I use it like

std::set<A*, comp> myset;

// Insert some data
A* a = new A(2,1);
A* b = new A(1,3);
myset.insert(a);
myset.insert(b);

Now my problem is that I would like to do this:

myset.find( (2 << 16) | 1 );

But, of course, it excepts A* not short int.

So, I know I could use std::find_if, but won't it render useless the custom comparator? It would iterate the whole list, wouldn't it? Is there any way I could use find with the hashCode rather than the object itself?

Thank you!

Answer 1

set::find takes argument of type key_type (see discussion Why is set::find not a template?). Using a std::set you have to construct a temporary object to use find.

myset.find(A(2, 1));

If A is not cheap to construct you might want to use a std::map<int, A> (or a wrapper around it) instead.