Constructors and Destructors

As classes have complex internal structures, comprising functions and data, cleanup and object initialization for classes is more perplexed than it is for simple data structures. Constructors and destructors are peculiar member functions of the classes that are employed to construct and destruct class objects. Construction might comprise initialization for objects and memory allocation. Destruction might comprise deallocation of memory  and cleanup for objects.

Similar to other member functions,destructors  and constructors are declared within  class declaration. They could be determined external  or inline to the class declaration. Constructors could have default arguments. Not like  other member functions, constructors could have member initialization lists. The accompanying confinements are applied to constructors and destructors are mentioned below:

ñ  Constructors and destructors do not have return types nor could they return values.

ñ  Pointers and References cannot be employed on constructors and destructors as their addresses could not be taken.

ñ  Constructors and destructors should not  be declared volatile,  const or static.

ñ  Unions shall not comprise class objects that have constructors or destructors.

ñ  Constructors shall be declared with the keyword virtual.

Constructors and destructors adapt the similar accession conventions as member functions. For illustration, if computer programmer declare the constructor with protected access, only derived classes and friends could employ it to produce class objects.

The compiler mechanically calls constructors when determining class objects and calls destructors when class objects move  out of scope. A constructor does not apportion memory for the class object its this pointer concerns to, but might apportion computer memory for more objects than its class object brings up. If memory allocation is called for for objects, constructors could in an explicit manner call the new operator. All the way through cleanup, the destructor might bring out objects apportioned by the representing constructor. To bring out objects, employ the delete operator.

Derived classes do not inherit constructors or destructors from their base classes, but they do call the constructor and destructor of base classes. Destructors could be declared with the keyword virtual. Constructors are also known as when temporary  or local class objects are produced and destructors are called when temporary  or local objects move  out of scope.

Computer programmer could call member functions from constructors or destructors. Computer programmer could call the virtual function, either indirectly or directly, from the constructor or destructor of the class A. In this event, the function known as is the one delimitated in A or the base class of A, but not the function overridden in any class derived from A. This averts the hypothesis of getting at the unconstructed object from the constructor or destructor

The main employment of constructors is to initialize objects. The function of initialization is automatically conveyed out by the employ of the special member function known as the constructor.

 General Syntax of Constructor

A constructor is the special member function that takes the similar name as the class name. The syntax in general is as given below:

{ arguments};

The default constructor for the class X has the form

X::X()

In the above illustration, the arguments are optional.

The constructor is automatically named when the object is created. A constructor is named whenever the object is outlined or dynamically allocated employing the "new" operator. Constructors are not automatically inherited among base and derived classes. If computer programmer don't supply one in the derived class, the default will be rendered but this might not do what computer programmer want. If no constructor is supplied then the default one is created by the compiler without any parameters. There must always be the constructor, even if it is the default and empty. If computer programmer supply the constructor with parameters then the default will NOT be created.

Detailed description about constructors:

ñ  Constructors are the functions with the similar name as of the class.

ñ  Constructors are proposed to initialize the members of  class when the instance of that class is created.

ñ  Constructors are not known as directly (take exception to through initializer lists)

ñ  Constructors are never virtual.

ñ  Multiple constructors for the similar class could be defined. They must have various parameters to differentiate them.

Destructors

Destructors are also special member functions employed in C++ programming language. Destructors have the opposite function of the constructor. The main employ of destructors is to release dynamic allocated memory. Destructors are employed to free memory, release resources and to execute other clean up. Destructors are automatically named when the object is destructed. Similar to  constructors, destructors also take the similar name as that of class name.

General Syntax of the Destructors is:

~ classname();

The above is the general syntax of the destructor. In the above, the symbol tilda ~ represents the destructor which precedes the name of the class. Some crucial points about destructors:

ñ  Destructors acquire the similar name as the class name.

ñ  Like the constructor, the destructor should also be defined in the public. The destructor must be public member.

ñ  The Destructor do not acquire any argument which refers that destructors cannot be overloaded.

ñ  There is no return type specified for destructors.

Destructors are by and large employed to free memory that was previously allocated and do other cleaning for the class object and its class members when the object is destructed. A destructor is called for the class object when that object moves out of scope or is in an explicit manner deleted. A destructor is the member function with the similar name as its class affixed by the ~ (tilde). For illustration:

class X {

public:

  // Constructor for class X

  X();

  // Destructor for class X

  ~X();

};

A destructor acquires no arguments and also has no return type. The address of the  destructor cannot be taken. Destructors cannot be declared, const volatile, volatile,  const  or static. A destructor could be declared pure virtual or virtual. If no user-defined destructor subsists for the class and one is called for, the compiler without ever expressing so clearly declares the destructor. This in implicit manner declared destructor is the inline public member of its class.

The compiler will in implicit manner define the in implicit manner declared destructor when the compiler uses the destructor to destruct the object of the destructor's class type. Suppose the class A has the in implicit manner declared destructor. The accompanying is tantamount to the function the compiler and will be  in implicit manner outlined for A:

  A::~A() { }

The compiler first in implicit manner outlines the in implicit manner declared destructors of the base classes and non static  data members of the class A before determining the in implicit manner declared destructor of A

A destructor of the class A is  small and of little importance if all the accompanying are true:

ñ  It is defined in  the implicit manner.

ñ  All  direct base classes of A shall have small and of little importance destructors.

ñ  The classes of all  non static data members of A should have small and of little importance destructors.

ñ  If any of the above are false, then destructor is non small and of little importance.

ñ  A union member cannot be of the class type that has the nontrivial destructor.

Class members that are class types could have their own destructors. Both base and derived classes could have destructors, in spite of the fact that destructors are not inherited. If the base class A or the member of A has the destructor, and the class derived from A does not declare the destructor, the default destructor is brought forth.

ñ  The default destructor calls  destructors of  base class and members of  derived class.

ñ  The destructors of base classes and members are known as in the reverse order of the concluding action of their constructor:

ñ  The destructor for the class object is known as before destructors for members and bases are known as.

ñ  Destructors for non static members are known as before destructors for base classes are known as.

ñ  Destructors for non virtual base classes are known as before destructors for virtual base classes are known as.

When the exception is thrown for the class object with the destructor, the destructor for the temporary object thrown is not known as before the time control communicates out of the catch block.

Destructors are in implicit manner known as when the automatic object  the local object that has been declared register or auto or not declared as extern or temporary object communicates out of scope. They are in implicit manner known as at program abortion for constructed static  and external objects. Destructors are aroused when computer programmer employ the delete operator for objects produced with the new operator.