Question

I have a function like this:

void something(void *obj)
{
    obj->Set();
}

The compiler says that left of the dereference operator has to be a pointer to a class/struct/union/generic (translated from german not sure about wording).

The idea is that I want to call the something function of obj no matter what is passed to something. It is ensured that it has this function. How can i achieve that?

–EDIT–

I started to work on an existing Software which has like > 100 Classes for datatypes. In one part of the code there is a big switch statement which depending on an id creates an instance of one of these classes and calls the Set function for that one. Now i want to do multiple of these calls parallel, and because of this i want to bring the ->Set() call to a seperate function which i then can call in a new thread. Sadly there is no baseclass and i cant change too much in the “big picture”. What is the best way to do this?

Answer 1

C++ doesn’t allow this (for good reasons: even if you can ensure that the object always has a function, C++ cannot, and since you can make mistakes, C++ is justified in distrusting you).

The proper way to do this is to have a common base class which defined this method for all types that you want to use here, and then use this common base class as the argument of this function.

Alternatively, if it’s known at compile time which type is used here, then the appropriate implementation uses templates:

template <typename T>
void f(T const& obj) {
    obj.something();
}

Whatever you do, void* is not appropriate. There are very rare legitimate use-cases for it in C++.