virtual base classes

In Marshals cline and Gregs Lomow's book C++ FAQ second edition FAQ21.11 says:
Can a pure virtual function be defined in the same class that declares it?
A: Yes, but new C++ programmers dont usually understand what it means, so this practice should be avoided... etc.

Can somebody give me an example of how one would define a pure virtual function?
thanks

In your base class:

virtual void MyPureVirtualFunction(void) = 0;

The voids can be any data type.

The use of a pure virtual function in the base class ensures that the complier will check for an override of that function in all the derived classes. If you forget to override it, it gives you a compile time error.

thanks for your post, but Im not looking for how to _declare_ a pure virtual function, im trying to understand what the C++ FAQs call _defining_ a virtual function in the same class that declares it. As in, you declare a pure virtual function like so:
virtual void Draw() =0;
can you define it? apparently so according to the book, how do you define it and why would you?
thanks

[Lynx]

Usually, if you declare pure virtual functions in a class, that class will be an abstract class and you can't directly declare an instance of an abstract base class. However, you can implement these pure virtual functions in the class, but it's not necessary because the inherited classes have the responsibility to implement them if they want to use these virtual functions.

If you do implement these pure virtual functions, you still can't declare an instance of this abstract class. But, the inherited class can call the functions.

For example:

class A{
public:
A(){}
~A(){}
virtual void foo1()=0; // pure virtual function
virtual void foo2()=0; // pure virtual function
virtual void foo3()=0; // pure virtual function
};
// class A only implement foo1()
void A::foo1(){printf("class A: foo1");}

class B : public A
{
public:
B(){}
~B(){}
virtual void foo1(){A::foo1();}
virtual void foo2(){printf("class B: foo2");}
virtual void foo3(){A::foo3();}
};
// class B only implement foo2

// if you ..
A objA; // compile error: class A is an abstract class

// if you ..
B objB; // link error: foo3() not found in A

// if foo3 isn't declared in class B
B objB; // OK
objB.foo1(); // output --> class A: foo1
objB.foo2(); // output --> class B: foo2
// end

Therefore, the implementation for foo1 in class A is not necessary; otherwise, foo1() doesn't need to be declared as a pure virtual function.

That's my opion. Good luck.

Lynx

Thanks that cleared things up.

I can see why they recomend not to do it though, cant think of a single usefull use of this, and theres many ways to make an error. Why not just dissalow defining pure virtual functions then? hmm