make_callback.cpp

#include <iostream>

template<typename Target, typename Source>
struct force_cast {
    union {
        Source source;
        Target target;
    };

    force_cast(Source source) : source(source) {};

    operator Target() {
        return target;
    }
};

template<typename Class, typename Ret, typename... Args>
Ret(*make_callback(Ret(Class::*source)(Args...)))(void *, Args...) {
    return force_cast<Ret(*)(void *, Args...), Ret(Class::*)(Args...)>(source);
}

using wanted_function = void(*)(void *self, int a);

void test_callback(wanted_function cb, void *self) {
    cb(self, 5);
}

class TestClass {
    void onEvent(int a) {
        std::cout << "a=" << a << std::endl;
    }
public:
    TestClass() {
        test_callback(make_callback(TestClass::onEvent), this);
    }
};

int main() {
    TestClass test;
}