Question

I’ve been developing in C++ for some time when I was a student, but I never used virtual class or extern in C++ in any of the projects. I just recent read about these two, and was hoping if someone had a better understanding of their usage.

What is the purpose of virtual class? An example of where it could be used/implemented. I gloss over it a bit on IBM website and wrote a test program to see it in action, but when would it be good to use a virtual class?

The same goes for extern as well. I saw an example, and did a test for myself in C++, but what is the advantage of using extern instead of using a header file? And what is the advantage of a header file instead of extern?

Answer 1

Virtual classes are for when you encounter the dreaded diamond. For example:

struct Base { int x; };
struct D1 : Base {};
struct D2 : Base {};
struct Derived : D1, D2 {};

Here, Derived actually has two Base parts, and as a result two member variables called x. It will compile, but you might experience some unexpected behaviour when manipulating a Derived object through one of its base classes.

Derived derived;
D1& d1 = derived;
D2& d2 = derived;
d1.x = 1;
d2.x = 2;
cout << d1.x << d2.x << endl; // 12 !

Virtual inheritance solves this problem by making Derived derive from Base only once.

struct Base { int x; };
struct D1 : virtual Base {};
struct D2 : virtual Base {};
struct Derived : D1, D2 {};

Here, Derived only has one Base part, and one member variable called x.