C++ – Pure Virtual functions

C++ Pure Virtual functions are used to create an abstract classes or interfaces. Pure Virtual functions have no function definition; just the function will be declared and the definition left to the derived classes.

In C++, like Java, there is no interface keyword to create an interface. To create an interface in C++, the option is to use Pure Virtual Functions.

Abstract class in C++

Any class that has one or more pure virtual functions, is called an abstract class or an interface class. C++ doesn’t allow to create objects or instances for an abstract class.

Then, how to access members from an abstract class? The solution is:

• Create a derived class from an abstract class
• Define all the pure virtual functions in derived class
• Create an object of derived class to access members of an abstract class

C++ abstract classes may be partially defined. That means, you can have some implemented functions and one ore more pure virtual functions inside the class. Once, all class members are completely defined, C++ allows to create an instance.

Here is the syntax to declare a Pure Virtual Function:

virtual function-return-type Function-Name(arguments) = 0;

Pure Virtual Functions must be declared as virtual with virtual keyword and it should be assigned to “0”; means, no definition to the function.

For example: Here, we are going to create an abstract class MyInterface with a Pure Virtual Function, Display. Display function is simply display the message.

Note: This program was compiled and tested using “Microsoft Visual Studio 2012”

Step 1. Define an abstract class MyInterface with Display, Pure Virtual Function.

class MyInterface
{
public:
	virtual void Display(_TCHAR* message) = 0;
};

Step 2. Create a derived class from MyInterface and implement it’s Pure Virtual Function, Display.

class SimpleClass : public MyInterface
{
public:
	void Display(_TCHAR* message)
	{
		_tprintf(message);
	}
};

Step 3. Create an instance of derived class and call the method, Display.

int _tmain(int argc, _TCHAR* argv[])
{
	SimpleClass obj;

	obj.Display(L"Hello, World!");

	return 0;
}

Once you run the application, you will see “Hello, World!” message on the screen.

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