C++: Operator overloading of < for pointers to objects

Question

Hello, I would like to compare two objects through their addresses. I tried operator overloading and it does not seem to work for pointers, but works for objects themselves. The following is the relevant code:

class C {
        public:
                int x;
};
.
.
.
bool operator <( C *ptr_c1, C *ptr_c2 )
{
        return ( (*ptr_c1).x < (*ptr_c2).x );
}

Answer 1

A pointer is a native C++ type and you can only overload operators for user defined types (i.e. classes).

If this was possible it would violate the principal of 'least surprise' and cause pointer comparisons to behave differently based on whether not your definition was visible. This would probably be very confusing.

If you want to use an alternative comparison for use in container or with algorithms, note that you can provide a replacement for the default std::less in most associative containers and algorithms that require a partial order.

Answer 2

if(*obj1 < *obj2) { /* this should use the overloaded operator. */ }

Answer 3

See these:

• Universal less<> for pointers in C++ standard
• What are the operations supported by raw pointer and function pointer in C/C++?

Answer 4

I tried operator overloading and it does not seem to work for pointers,

Correct.

but works for objects themselves.

Correct.

So what is the question?

Do you want to use pointers in a sorted container?

#include <iostream>
#include <set>

struct X
{
    int x;
};

struct XPTest
{
    bool operator()(X* const& lhs,X* const& rhs)
    {
        return lhs->x < rhs->x;
    }
};

int main()
{
    X       x1; x1.x    = 5;
    X       x2; x2.x    = 6;

    std::set<X*,XPTest> plop;

    plop.insert(&x1);
    plop.insert(&x2);
}

Answer 5

Thank you very much for the suggestions. Now I realize I should have been a bit more specific for the need to say why I cannot use the beautiful solution suggested by KiNgMaR.

I am actually trying to keep track of objects in a STL priority_queue and update some internals while they are in the queue. (I would not change the priority itself.) But priority_queue works on operator overloading and when I tried that C++ objected. :(

I guess I will have to write my own complete code here implementing heaps and maps.