C++ Inheritance

The main thrust in a C++ project is toward developing classes that solve a given problem. These classes are generally built incrementally starting from basic, simple classes, using inheritance.

When creating a new class, instead of writing completely new data members and member functions, the programmer can designate that the new class is to inherit the data members and member functions of a previously defined base class, sometimes called the parent class. When inheriting features from a parent, you can make the derived class extend them, restrict them, change them, eliminate them, or use them without any changes. All these variations can be grouped into two basic object-oriented techniques. The first is called feature restriction, which limits or eliminates a feature of the parent. The second is called feature expansion, which adds something to a parent feature.

Anytime you build a new class, you should determine wheather you can use a pre-existing class a a base class.

In C++ not everything can be passed on through inheritance. A few special cases are inconsistent by definition with inheritance. The following cannot be inherited:

• Constructors: Derived classes automatically invoke the constructor of the base class when they are instantiated but after they are constructed, the base class constructor is off limits! A base class construtor cannot be invoked explicitly in a derived class.
• Destructors: Explicit invocation of a destructor is not allowed in a program, even in its own class.
• User-defined new operators: An overloaded new operator is considered static by the compiler, even without an explicit declaration. This means the operator is invoked before the constructor!
• User-defined assignment operators: An assignment operator is considered to be tightly bound to a class, much like the class constructor and destructor.
• friend relationships: Any friend relationships declared in classes of an inheritance tree do not propagate up or down in the tree.

When a class is derived from a base class, all the names in the base class automatically become private in the derived class. This is the default behavior in C++, but it can be changed using base class access specifiers. There are three of these:

• private This is the default specifier if none is specified when a class is declared. All the inheritable (that is, protected and public) names in the base class are private in the derived class.
• protected The public and protected names in the base class are protected in the derived class.
• public All the public names in the base class are public in the derived class, and all the protected names in the base class are protected in the derived class.