The characteristic feature of OOP is the encouragement of code reuse through inheritance. A new class is made from the existing, which is called the base class. The derived class uses the members of the base class, but can also modify and supplement them.
Many types are variations of the existing types. It is often tedious to develop a new code for each of them. In addition, the new code implies new errors. The derived class inherits the description of the base class, thus any re-development and re-testing of code is unnecessary. The inheritance relationships are hierarchical.
Hierarchy is a method that allows to copy the elements in all their diversity and complexity. It introduces the objects classification. For example, the periodic table of elements has gases. They possess to properties inherent to all periodic elements.
Inert gases constitute the next important subclass. The hierarchy is that the inert gas, such as argon is a gas, and gas, in its turn, is part of the system. Such a hierarchy allows to interpret behaviour of inert gases easily. We know that their atoms contain protons and electrons, that is true for all other elements.
We know that they are in a gaseous state at room temperature, like all the gases. We know that no gas from inert gas subclass enters usual chemical reaction with other elements, and it is a property of all inert gases.
Consider an example of the inheritance of geometric shapes. To describe the whole variety of simple shapes (circle, triangle, rectangle, square etc.), the best way is to create a base class (ADT), which is the ancestor of all the derived classes.
Let's create a base class CShape, which contains just the most common members describing the shape. These members describe properties that are characteristic of any shape - the type of the shape and main anchor point coordinates.
Next, create new classes derived from the base class, in which we will add necessary fields, each specifying a certain class. For the Circle shape it is necessary to add a member that contains the radius value. The Square shape is characterized by the side value. Therefore, derived classes, inherited from the base class CShape will be declared as follows:
For the Square shape class declaration is similar:
It should be noted that while object is created the base class constructor is called first, and then the constructor of the derived class is called. When an object is destroyed first the destructor of the derived class is called, and then a base class destructor is called.
Thus, by declaring the most general members in the base class, we can add an additional members in derived classes, which specify a particular class. Inheritance allows creating powerful code libraries that can be reused many times.
The syntax for creating a derived class from an already existing one is as follows:
One of aspects of the derived class is the visibility (openness) of its members successors (heirs). The public, protected and private keywords are used to indicate the extent, to which members of the base class will be available for the derived one. The public keyword after a colon in the header of a derived class indicates that the protected and public members of the base class CShape should be inherited as protected and public members of the derived class CCircle.
The private class members of the base class are not available for the derived class. The public inheritance also means that derived classes (CCircle and CSquare) are CShapes. That is, the Square (CSquare) is a shape (CShape), but the shape does not necessarily have to be a square.
The derived class is a modification of the base class, it inherits the protected and public members of the base class. The constructors and destructors of the base class cannot be inherited. In addition to members of the base class, new members are added in a derivative class.
The derived class may include the implementation of member functions, different from the base class. It has nothing common with an overload, when the meaning of the same function name may be different for different signatures.
In protected inheritance, public and protected members of base class become protected members of derived class. In private inheritance, the public and protected members of base class become private members of the derived class.
In protected and private inheritance, the relation that "the object of a derivative class is object of a base class" is not true. The protected and private inheritance types are rare, and each of them needs to be used carefully.
It should be understood that the type of inheritance (public, protected or private) does not affect the ways of accessing the members of base classes in the hierarchy of inheritance from a derived class. With any type of inheritance, only base class members declared with public and protected access specifiers will be available out of the derived classes. Let's consider it in the following example:
In the above example, CBaseClass has only a public method - the constructor. Constructors are called automatically when creating a class object. Therefore, the private member m_member and the protected methods Member() cannot be called from the outside. But in case of public inheritance, the Member() method of the base class will be available from the derived classes.
In case of protected inheritance, all the members of the base class with public and protected access become protected. It means that if public data members and methods of the base class were accessible from the outside, with protected inheritance they are available only from the classes of the derived class and its further derivatives.
The example shows that methods SetPI() and GetPi() in the base class CBaseMathClass are open and available for calling from any place of the program. But at the same time, for CProtectedChildClass which is derived from it these methods can be called only from the methods of the CProtectedChildClass class or its derived classes.
In case of private inheritance, all the members of the basic class with the public and protected access become private, and calling them becomes impossible in further inheritance.
MQL4 has no multiple inheritance.