| C++ Neural Networks and Fuzzy Logic |
by Valluru B. Rao
M&T Books, IDG Books Worldwide, Inc.
ISBN: 1558515526 Pub Date: 06/01/95
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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.
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.
Encapsulation of data and functions would also allow you to use the same function name in two different objects. The use of a name for a function more than once does not have to be only in different object declarations. Within the same object one can use the same name for functions with different functionality, if they can be distinguished in terms of either their return type or in terms of their argument types and number. This feature is called overloading. For example, if two different types of variables are data items in an object, a commonly named function can be the addition, one for each of the two types of variablesthus taking advantage of overloading. Then the function addition is said to be overloaded. But remember that the function main is just about the only function that cannot be overloaded.
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.
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