C++ forbid overwriting a virtual function

I use a class A from a library and want to add some functionality to it via an own class B. The user of c开发者_如何转开发lass B should derive from it as if he would derive from class A.

class A {
    public:
        virtual void func1()=0;
        virtual void func2()=0;
        ...
}
class B: public A {
    public:
        virtual void func1() {...}
}

So if someone creates a class C deriving from B, he should have to implement func2:

class C: public B {
    public:
        virtual void func2() {...}
}

It is very important for my application, that class C doesn't overwrite func1, eliminating B::func1().

Is there a way to forbid overwriting this virtual function for all child classes of B? If not in plain C++, is there something in boost MPL that throws a compiler error, when this function is overwritten?

No, that's not possible in the current edition of C++, aka C++03. The upcoming C++11 standard will include the contextual keyword final which will make this possible:

// C++11 only: indicates that the function cannot be overridden in a subclass
virtual void MemberFunction() final { ... }

The Microsoft Visual C++ compiler also includes the keyword sealed, as an extension, which functions similarly to the C++11 keyword final, but this only works with Microsoft's compiler.

Not in C++03, but C++0x provides the special "final" identifier to prohibit this:

http://en.wikipedia.org/wiki/C++0x#Explicit_overrides_and_final

No. In C++03, you cannot stop derived classes from overriding¹ virtual function(s) of base classes. However, base classes can force the derived (non-abstract) classes to provide implementation for virtual functions (in which case, the virtual functions are actually pure virtual functions).

^{1. The correct terminology is override, not overwrite.}