Polymorphism in C++

The correct solution is to apply the virtual keyword. The direct effect of this on the pointed function is that, on functions from the base classes, rather than being redefined, it will be overwritten.

We will declare as virtual our function inside the base class.

virtual bool appendToStream(stringstream& inTo) const;

Once you declare a function virtual, it will remain virtual in all of the derived classes and the classes derived from the derived class; once virtual, always virtual, to sum it up. The real strength in this solution is that, for all functions declared like this in the base class, when we have a base pointer pointing to a derived object, the called function will be declared.

By just adding the virtual keyword in front of the appendToStream function declaration, the same code entered inside the main function at the end of the Pointers segment of this article will now call the derived objects class function, and produce a very different result:

2

CarContainer

Car Type: Lamborghini Murcielago

Car Value : 313000

Press any key to continue . . .

Now we have the general usage. Imagine the same when we have a stream of incoming pointers to derived classes through base class pointers; for instance, a vector of base class pointers. If we decide to add a new class, if we overwrite for the virtual function, we do not need to modify a single character in the vector handling. All of this will be done by the compiler in the background. Here is a fast example:

BananaContainer banana(1, 1, 50);

CarContainer car(2, "Lamborghini Murcielago", 313000);

EmptyContainer empty(3);

stringstream inTo;

std::vector<BaseContainer*> allContainer;

allContainer.push_back(&banana);

allContainer.push_back(&car);

allContainer.push_back(&empty);

std::vector<BaseContainer*>::iterator it, end;

for (it = allContainer.begin(), end = allContainer.end();

it != end; ++it)

{

(*it)->appendToStream(inTo);

}

cout << inTo.str() << endl;

We can also decide to not overwrite the virtual function, as by default in this scenario the base class's function will be used. This is the case of the EmptyContainer, for which I decided to not write this function, and as follows, only the ID of the container is printed on the screen of your computer. In addition, the following appears on the console:

1

BananaContainer

Price per box: 1

Number of boxes 50

2

CarContainer

Car Type: Lamborghini Murcielago

Car Value : 313000

3

Press any key to continue . . .

Here, in a downloadable form, is the code for all of this written in Visual Studio 2008. It will work under any C++ compiler as long as you build a new empty console project and add these files to it. Be aware that it already contains some stuff that will be covered in the next article, so if you happen to bump into something that is not covered here, wait a little while. Nevertheless, feel free to try the things I have shown you in this article.

Take the time to understand the content I offered to you today. I've just scratched the surface of polymorphism and virtual functions. Feel free to post your comment here on the blog.

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