Why can't I compare a constant instance of a class with a non-constant instance of the same class?

Question

I have the following function:

#include <cstdint>
using int_32 = int32_t;

bool func(const Coord<int_32> c)
{
    for (Coord<int_32> i : boxes)
        if (c == i)
            return 0;
    return 1;
}

Coord<int_32> is a struct with two members of type int ( int32_t ).

Here is its overloaded == operator:

bool operator == (const Coord<T> &p)
{
    if (this -> x == p.x &&
        this -> y == p.y)
        return 1;
    return 0;
}

It gives me an error at if (c == i) :

error: passing 'const Coord<int>' as 'this' argument discards qualifiers [-fpermissive]

Answer 1

In your comparison:

if (c == i)

the variable c is const . However, your operator== is not const-qualified , so the left hand side of == is required to be non-const.

The correct fix for this is to mark operator== as const if it's a member function:

bool operator == (const Coord<T> &p) const {
                                 //  ^^^^^