Cast pointer to member function to normal pointer

Question

Currently I have a class of this kind, shortened for simplicity:

class MyClass {
    public: 
        MyClass();
        void* someFunc(void* param);
}

Now I need to call a function of this kind (not member of any class and which I unfortunately cannot change) but which I need to call anyway:

void secondFunc(int a, int b, void *(*pCallback)(void*));

Now I need to pass the address of someFunc of an instance.

A not working sample:

MyClass demoInstance;
// some other calls
secondFunc( 1, 2, demoInstance::someFunc() );

I've tried also with casts like:

(void* (*)(void*)) demoInstance::someFunc;
reinterpret_cast<(void* (*)(void*))>(demoInstance::someFunc);

How can I call this function with a class' member function as parameter so that this one can use it as callback?

Any idea or remark is appreciated. Thanks and regards tobias

Answer 1

You can't call the member function directly. Member function pointers are not the same type as function pointers.

You'll need to wrap it in a compatible function somehow. However, if your outer function (the one taking the function pointer as an argument) is not re-entrant and does not supply an extra argument for use by the function pointer, you won't be able to pass the instance upon which the member function operates, so you won't actually be able to make the call.