C++ class pointer

Question

I know that you can get a reference to a static method like this:

typedef void (*pointer)();
pointer p = &MyClass::MyMethod;

But is there a way to get a reference to the class itself?

EDIT: I'd like to store a class name to a variable so i could instantiate different kinds of objects based on the current value of the variable to a buffer or a list. I could then perform operations on the list of objects (using polymorphism).

Answer 1

No, "the class itself" is not an object that exists at runtime in C++ -- it's a compile-time concept only. Therefore you cannot get a reference or pointer to "a class object" per se, only to instances, functions (including static ones), and so on -- things that do exist at runtime.

With RTTI you can get somewhat-similar effects, though -- the typeid keyword gives you a reference to a std::type_info (you need to #include <typeinfo> to enable this) which "stands for" the type in question at runtime. (You may also need special compiler switches to enable this functionality, e.g. /GR for Microsoft's Visual C++).

Whether that's good enough for you depends on exactly what it is that you're trying to accomplish. Care to clarify?

Answer 2

No, classes are not first-class object in current versions of C++ (unlike other languages like Java, Python, etc.).

Answer 3

What you want to accomplish can be done by taking a function pointer, if by no other means then at least like this:

class Base {
}

class A : public Base {
}

class B : public Base {
}

Base *ConstructA() { return new A(); }
Base *ConstructB() { return new B(); }

Base (* c)();

...
     c = ConstructB;
     list <Base *> mylist;
     mylist.push_back(c());

Answer 4

Although pure C++ doesn't have notions of class at run-time, there are libraries that provide such functionality.

Qt has a meta-object system that provides run-time class reflection/introspection, dynamic properties, and async communication.

http://qt-project.org/doc/qt-4.8/qmetaobject.html#details

Also, CERN's ROOT provides similar capabilities. See TObject and TClass.

http://root.cern.ch/drupal/content/reflex

Both are cross-platform and fairly tool chain compatible. For larger and more complex projects especially, something like these can remove many of the drawbacks of using C++ compared to other languages.

Answer 5

What you need is a factory. You can pass it a class name and it will return a pointer to a (new) instance of that class. But this isn't magic; you'll have to write the methods yourself.

When you talk about storing objects of different types in a list, I assume you mean you're going to store pointers to the objects, not the objects themselves (otherwise you're going to have real trouble). And if you're going to do that without violence, and also use polymorphism, these classes had better be derived from a common base.

If and when they are, and the constructors you want to use have similar signatures, you can use templates to save yourself some work:

class myFactory
{
    template <class T>
    Base * makeThing(int n, string s) // whatever arguments you like
    {
            T * ret = new T(n, s);
            return(dynamic_cast<Base*>(ret));
    }
};