Question

How should I pass a function inside an struct as a functor? I assumed this should work fine, but it didn’t:

#include <algorithm>
using namespace std;

struct s {
    int a[10];

    bool cmp(int i, int j) {
        // return something
    }

    void init() {
        sort(a, a + 10, cmp);
    }
};

which gets <unresolved overloaded function type>

Answer 1

You cannot do this directly, because cmp is a member function, which requires three arguments: i, j, and the invisible, implicit this pointer.

To pass cmp to std::sort, make it a static function, which does not belong to any particular instance of s and thus doesn’t have a this pointer:

static bool cmp(int i, int j) {
    // return something
}

---

If you need access to this, you can wrap cmp in a simple function object instead:

struct cmp {
    s &self;
    cmp(s &self) : self(self) { }
    bool operator()(int i, int j) {
        // return something, using self in the place of this
    }
};

And call it like this:

sort(a, a + 10, cmp(*this));