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Wednesday, June 20, 2007

Destructor Functions in Detail

If you don’t know what destructor functions are, read Introduction to Constructor and Destructor functions of a Class. As the example programs in this article makes use of the dynamic memory allocation of C++, so please read Introduction to Dynamic Memory Allocation in C++ , in case you missed it.

When does the destructor function gets invoked

The examples below illustrates when the destructor function gets invoked:

Example 1:

  //Example Program in C++
  #include<iostream.h>

  class myclass
  {
   public:

   ~myclass()
     {
      cout<<"destructed\n";
     }
  };

  void main(void)
  {
   myclass obj;
   cout<<"inside main\n";
  }

OUTPUT:

   inside main
   destructed
   Press any key to continue

As I said in the other article, destructors get invoked when the object of a class goes out of scope. In this case, the object goes out of scope as the program terminates. So the destructor gets invoked just before the program’s termination.

Example 2:

  //Example Program in C++
  #include<iostream.h>

  void myfunc(void);
  class myclass
  {
   public:

   ~myclass()
     {
      cout<<"destructed\n";
     }
  };

  void main(void)
  {
   cout<<"inside main\n";
   myfunc();
   cout<<"again inside main\n";
  }

  void myfunc(void)
  {
   cout<<"inside myfunc\n";
   myclass obj;
   cout<<"still inside myfunc\n";
  }

OUTPUT:

   inside main
   inside myfunc
   still inside myfunc
   destructed
   again inside main
   Press any key to continue

In this case, destructor function is invoked just as the program’s execution returns from the function, but before executing any further instruction from where it was called (main).

Example 3: In the following example we are creating a dynamically allocated object of a class in the same way as we did with the variables.

  //Example Program in C++
  #include<iostream.h>

  class myclass
  {
   public:
   ~myclass()
     {
      cout<<"destructed\n";
     }
  };

  void main(void)
  {
   myclass *obj;
   obj=new myclass;
   cout<<"inside main\n";

   delete obj;

   cout<<"still inside main\n";
  }

OUTPUT:

   inside main
   destructed
   still inside main
   Press any key to continue

Here the programmer is explicitly destroying the object, hence the destructor function is called.

Now that you know some details about the destructor function, let me give you a practical example of the use of destructor function:

  //Example Program in C++
  #include<iostream.h>

  class myclass
  {
   int *number;

   public:
   myclass(int num)
     {
      number=new int[num];
     }

   ~myclass()
     {
      delete []number;
     }

   void input_num(int index, int num)
     {
      number[index]=num;
     }

   int output_num(int index)
     {
      return number[index];
     }
  };

  void main(void)
  {
   int size, num;
   cout<<"enter number of elments: ";
   cin>>size;

   myclass obj(size);

   for(int i=0;i<size;i++)
     {
      cout<<"enter element "<<i+1<<":";
      cin>>num;
      obj.input_num(i,num);
     }

   cout<<"\nelements have the following values\n\n";

   for(i=0;i<size;i++)
     {
      cout<<"element "<<i+1<<":";
      cout<<obj.output_num(i);
      cout<<"\n";
     }
  }

Good-Bye!

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