How to use a static method as a callback in c++

December 4, 2021

I have a comparison/ordering function that relates to a class. I can use it if I define it as a separate closure object. I would like to make it into a static method of the class it operates on so it is tidier. I guessed how to do this but I get an error that I can’t interpret.

Generally I would like to know how to treat static methods as callable objects.

Minimal related example code (not working):

#include <set>

class MyClass {

    // More code here

    static auto compare(MyClass a, MyClass b)   {
        return a < b;
    }
};

int main() {
    std::set<MyClass, decltype(MyClass::compare)> s(MyClass::compare);
    return 0;
}

Update, I’m not sure if my example confused the issue, so I updated it.

>Solution :

Couple of issues:

compare is private, make it public.
One must use & to get the address of functions.

#include <set>

class MyClass {
public:
    static auto compare(int a, int b)   {
        return a < b;
    }
};

int main() {
    std::set<int, decltype(&MyClass::compare)> s(&MyClass::compare);
    return 0;
}