Difference between base class and derived class?

Answer 1

The derived class derives, or inherits, from the base class. The derived class is like a "child", and the base class, the "parent". The child class has the attributes of the parent class - its variables (those defined at the class level) and methods. Changes done to the parent class (the base class) will affect the child class (the derived class).

Answer 2

A base class is a class from which the least-derived class inherits directly and from which the most-derived class inherits indirectly from the least-derived class. A virtual base class is a class from which the most-derived class inherits directly while the least-derived class inherits directly but virtually via the most-derived class.

Example of non-virtual base class declaration

class base{};

class least_derived : public base{};

class most_derived : public least_derived{};

Here, the most_derived class inherits the baseclass, indirectly, via the least_derived class, which inherits base class directly.

Example of virtual base class declaration

class base{};

class least_derived : virtual public base{};

class most_derived : public least_derived{};

Here, the most_derived class inherits the baseclass directly, while least_derived class inherits base class virtually (via the most_derived class). If there is no most_derived class, the least_derivedclass will inherit base class directly (just as if it were non-virtual).

In single inheritance, such as shown above, there is very little difference in terms of behaviour. Virtual base classes become far more useful when used with multiple inheritance.

Example of non-virtual base class declaration in multiple inheritance

class base{};

class least_derived_1 : public base{};

class least_derived_2 : public base{};

class most_derived : public least_derived_1, public least_derived_2 {};

Here, most_derived inherits two instances of baseclass, indirectly, one from least_derived_1 and the other from least_derived_2. This introduces an ambiguity when referring implicitly to the base class, meaning you must access the appropriate base class explicitly. E.g., most_derived.least_derived1::base or most_derived.least_derived2::base.

Example of virtual base class declaration in multiple inheritance

class base{};

class least_derived_1 : virtual public base{};

class least_derived_2 : virtual public base{};

class most_derived : public least_derived_1, public least_derived_2 {};

Here, most_derived inherits just one instance of base class, directly, while least_derived_1 and least_derived_2 share that same instance, virtually. This removes all ambiguity and allows you to access the baseclass implicitly from the most_derived class. E.g., most_derived::base.

Note that not all base classes can be inherited virtually. Ideally, the base class should have few member variables (preferably none at all), and should be an abstract base class (a class with at least one pure-virtual method). Since all derivatives effectively share the same instance of the base class, this not only removes all ambiguity, but also reduces the memory footprint of the most-derived class.