Abstract classes act as expressions of general concepts from which more specific classes can be derived. You cannot create an object of an abstract class type; however, you can use pointers and references to abstract class types.
A class that contains at least one pure virtual function is considered an abstract class. Classes derived from the abstract class must implement the pure virtual function or they, too, are abstract classes.
A virtual function is declared as “pure” by using the pure-specifier syntax (described in Class Protocol Implementation). Consider the example presented in Virtual Functions. The intent of class Account is to provide general functionality, but objects of type Account are too general to be useful. Therefore, Account is a good candidate for an abstract class:
class Account
{
public:
Account( double d ); // Constructor.
virtual double GetBalance(); // Obtain balance.
virtual void PrintBalance() = 0; // Pure virtual function.
private:
double _balance;
};
The only difference between this declaration and the previous one is that PrintBalance is declared with the pure specifier (= 0).