Object- Oriented Programming (OOP) is an approach to program organization and development that attempts to eliminate some of the pitfalls of conventional programming methods by in incorporating the best of structured programming features with several powerful new concepts. It is a new way of organizing and developing programs and has nothing to do with any particular language. However, not all languages are suitable to implement the OPP concepts easily.
Object Oriented Programming was developed because limitations were discovered in earlier approach to programming. To appreciate what OOP does, first all discuss what these limitations one and how they arose from traditional programming
The C++ Language
C++ can be viewed as a procedural language with some additional constructs, some of which are added for object oriented programming and some for improved procedural syntax. A well written C++ program will reflect elements of both object oriented programming style and classic procedural programming. C++ is actually an extendible language since we can define new types in such a way that they act just like the predefined types which are part of the standard language. C++ is designed for large scale software development.
Provided by the ANSI-C standard. As the ANSI-C standard was in development, many of the newer constructs from C++ were included as parts of C itself, so even though C++ is a derivation and extension of C, it would be fair to say that ANSI-C has some of its roots in C++. An example is prototyping which was developed for C++ and later added to C.
C++ & Object Orientation
C++ provides the programmers with all tools to implement Object Oriented Concepts in their programs. A programmer can
- Use public and private access specifiers to restrict the access to the data, objects and functionality
- Can completely hide the sensitive from the unauthorized access Can use multiple inheritance
And many more features that a programmer can use to program. Due to this functionality of C++ Programming Language it is the one of the most used programming language in any Operating system platform.
In C++ you have the facility to encapsulate data and the operations that manipulate that data, in an appropriate object. This enables the use of these collections of data and function, called objects , in programs other than the program for which they were originally created. With objects, just as with the traditional concept of subroutines, you make functional blocks of code. You still have language-supported abstractions such as scope and separate compilation available. This is a rudimentary form of encapsulation. Objects carry encapsulation a step further. With objects, you define not only the way a function operates, or its implementation, but also the way an object can be accessed, or its interface. You can specify access differently for different entities. For example, you could make function do_operation() contained inside Object A accessible to Object B but not to Object C. This access qualification can also be used for data members inside an object. The encapsulation of data and the intended operations on them prevents the data from being subjected to operations not meant for them. This is what really makes objects reusable and portable! The operations are usually given in the form of functions operating upon the data items. Such functions are also called methods in some object-oriented programming languages. The data items and the functions that manipulate them are combined into a structure called a class. A class is an abstract data type. When you make an instance of a class, you have an object. This is no different than when you instantiate an integer type to create variables i and j. For example, you design a class called ElectronicBook, with a data element called ArrayofPages. When you instantiate your class you make objects of type ElectronicBook.
Suppose that you create two of these called EB_Geography and EB_History. Every object that is instantiated has its own data member inside it, referred to by ArrayOfPages.
Related to the idea of encapsulation is the concept of data hiding. Encapsulation hides the data from other classes and functions in other classes. Going back to the ElectronicBook class, you could define functions like GetNextPage, GetPreviousPage, and GetCurrentPage as the only means of accessing information in the Array of Pages data member, by functions that access the ElectronicBook object. Although there may be a hundred and one other attributes and data elements in the class ElectronicBook, these are all hidden from view. This makes programs more reliable, since publishing a specific interface to an object prevents inadvertent access to data in ways that were not designed or accounted for. In C++, the access to an object, and its encapsulated data and functions is treated very carefully, by the use of keywords private, protected, and public. One has the opportunity
to make access specifications for data objects and functions as being private, or protected, or public while defining a class. Only when the declaration is made as public do other functions and objects have access to the object and its components without question. On the other hand, if the declaration happens to be as private, there is no possibility of such access. When the declaration given is as protected, then the access to data and functions in a class by others is not as free as when it is public, nor as restricted as when it is private. You can declare one class as derived from another class, which will be discussed shortly. So-called derived classes and the declaring class do get the access to the components of the object that are declared protected. One class that is not a derived class of a second class can get access to data items and functions of the second class if it is declared
as a friend class in the second. The three types of declarations of access specification can be different for different components of an object. For example, some of the data items could be declared public, some private, and the others protected. The same situation can occur with the functions in an object. When no explicit declaration is made, the default specification is as private.
Constructors and Destructors as Special Functions of C++
Constructors and destructors are special functions in C++. They define how an object is created and destroyed. You cannot have a class defined in a C++ program without declaring and defining at least one constructor for it. You may omit declaring and then defining a destructor only because the compiler you use will create a default destructor. More than one constructor, but only one destructor, can be declared for a class.
Constructors are for the creation of an object of a class and for initializing it. C++ requires that every function has a return type. The only exceptions are constructors and destructors. A constructor is given the same name as the class for which it is a constructor. It may take arguments or it may not need them. Different constructors for the same class differ in the number and types of arguments they take. It is a good idea to provide for each class at least a default constructor that does not take any arguments and does not do anything except create an object of that class type. A constructor is called at the time an object of its class is needed to be created.
A destructor also is given the same name as the class for which it is a destructor, but with the tilde (~) preceding the name. Typically, what is done in a destructor is to have statements that ask the system to delete the various data structures created for the class. This helps to free-up allocated memory for those data structures. A destructor is called when the object created earlier is no longer needed in the program.
Dynamic Memory Allocation
C++ has keywords new and delete, which are used as a pair in that order, though separated by other statements of the program. They are for making dynamic allocation of memory at the time of creation of a class object and for freeing-up such allocated memory when it is no longer needed. You create space on the heap with the use of new. This obviates the need in C++ for malloc, which is the function for dynamic memory allocation used in C.
Polymorphism and Polymorphic Functions
A polymorphic function is a function whose name is used in different ways in a program. It can be also declared virtual, if the intention is late binding. This enables it to be bound at run time. Late binding is also referred to as dynamic binding. An advantage in declaring a function in an object as virtual is that, if the program that uses this object calls that function only conditionally, there is no need to bind the function early, during the compilation of the program. It will be bound only if the condition is met and the call of the function takes place. For example, you could have a polymorphic function called draw() that is associated with different graphical objects, like for rectangle, circle, and sphere. The details or methods of the functions are different, but the name draw() is common. If you now have a collection of these objects and pick up an arbitrary object without knowing exactly what it is (via a pointer, for example), you can still invoke the draw function for the object and be assured that the right draw function will be bound to the object and called.
You can overload operators in addition to overloading functions. As a matter of fact, the system defined left shift operator << is also overloaded in C++ when used with cout, the C++ variation of the C language printf() function. There is a similar situation with the right shift operator >> in C++ when used with cin, the C++ variation of the C language scanf() function. You can take any operator and overload it. But you want to be cautious and not overdo it, and also you do not create confusion when you overload an operator. The guiding principle in this regard is the creation of a code-saving and time-saving facility while maintaining simplicity and clarity. Operator overloading is especially useful for doing normal arithmetic on nonstandard data types. You could overload the multiplication symbol to work with complex numbers, for example.