Constructor and Destructor in C++

Introduction of Constructor and Destructor in C++ 

• With the help of a constructor and destructor in C++, we can initialize and destroy the memory occupied by variables/data of user-defined data type (class) by using a specialized function as a constructor and destructor in object-oriented programming.
• In Constructor and Destructor in C++, the Constructor is used to create the object in an object-oriented programming language while the destructor is used to destroy the object.
• Constructors and destructors in C++ have no return type, not even void.
• Constructor and destructor in C++ are built-in member functions and customized member functions.

Constructor in C++

• C++ is an object-oriented programming (OOP) language that provides a special member function called a constructor for initializing an object when it is created. This is known as the automatic initialization of objects.
• The constructor has the same name as that of the class’s name.
• Its main job is to initialize the value of the class.
• Whenever an object is created in a class, the special member function (constructor) is executed automatically to allocate the memory for the newly created object.
• If a class has a constructor, each object of that class will be initialized automatically.
• A constructor for a class is needed so that the compiler automatically initializes an object as soon as it is created.
• If a class has no user-defined constructor exists then the compiler implicitly declares a default parameter constructor. This system-defined constructor is an inline public member of its class. When the compiler uses this type of constructor to create an object for a class then the constructor will have no constructor initializer and have a null body.

Characteristics/Features of Constructor

The constructor has some special characteristics which are as follows: –

• Constructor is declared in the public section of a class.
• Constructors are invoked/called directly when an object is created.
• Like other C++ functions, constructors may have default arguments.
• A constructor can be inside the class definition or outside the class definition.
• A class may have one or more constructors as needed and hence overloaded.

Limitations/Demerits of Constructor

The destructor has also some limitations which are as follows: –

• Constructors don’t have a return type, not even void, and hence can’t return any values.
• We cannot have two/more constructors with similar arguments.
• We cannot declare a virtual constructor hence we cannot call virtual methods from the constructor.
• The constructor has to compile time-bound, not dynamic/run time.
• They can’t be used in the union.
• Constructor can’t be a friend function.
• Constructors can’t be inherited through a derived class but can call the base class constructor.

Types of Constructor

There are the following types of constructors – 

(A) Default Constructor

• A constructor that either has no parameters or parameters with default/constant values.
• Using the default constructor, we can declare only instances of the class in an application.
• No default constructor is created for a class that has any constant or reference type members.

(B) Parameterized Constructor

• A constructor that can take arguments for its processing is called a parameterized constructor.
• When a constructor is parameterized, we must provide appropriate arguments for the constructor.

(C) Copy Constructor

• A copy constructor is used to declare and initialize an object from another defined object.
• The process of initializing an object’s value through a copy constructor is known as copy initialization.
• A copy constructor is always used when the compiler has to create a temporary object of a class object.
• The copy constructor takes an object of their own class as arguments and produces such an object.
• The copy constructors usually do not return a function value.
• The copy constructors are used in the following situations
  • The initialization of an object by another object of the same class.
  • Return of objects as a function value.
  • Stating the object as by value parameters of a function.

Constructor Overloading

• When more than one constructor is defined inside a class, it is called constructor overloading.
• In other words, when a class uses multiple constructors in a class for a specific purpose, it is called constructor overloading.
• It is used to increase the flexibility of a class by having more number constructors for a single class.
• It gives us more than one way to initialize objects in a class.

Destructor in C++

• A destructor is another specialized member function that is used to destroy the object’s memory when it is no longer required.
• A destructor is a special member function that has the same name as that of the class but is prefixed with a ~ (tilde) symbol.
• A destructor is defined under the public section of the class so that its objects can be destroyed in any function.
• Destructors are functions that are complementary to constructors.
• A destructor is used to destroy the objects that have been created by using a constructor. 
• They de-initialize objects when they are destroyed.
• A destructor is invoked/called when an object of the class goes out of scope, or when the memory occupied by it is de-allocated using the delete operator.
• It is called automatically by the compiler when an object is destroyed.
• There’s only one destructor for each object.

Characteristics/Features/Applications of Destructor

The main application and characteristics of destructors are given as follows: –

• Destructor functions are invoked automatically when the objects are destroyed.
• We can have only one destructor for a class, i.e. destructors can’t be overloaded.
• If a class has a destructor, each object of that class will be de-initialized before the object goes out of the scope.
• It is necessary for memory regeneration consumed by the constructor. In other words, a destructor cleans up the memory that is no longer required or accessible.
• The destructor function also obeys the usual access rules as other member functions do.
• Member function may be called from within a destructor.
• Destructors are less complicated than constructors.
• A destructor can be declared virtual or purely virtual.
• An object of a class with a destructor can’t be a member of a union.

Limitations/Demerits of Destructor

• It does not return any value, not even void.
• A destructor takes no arguments and has no return type.
• It is not called explicitly.
• It is not possible to take the address of the destructor.
• Destructors cannot be declared const, volatile, const volatile, or static.
• Destructors can’t be inherited.
• They are case-sensitive.
• They are not suitable for large & complex programs having thousands of lines of code.