How to use a destructor

This is a discussion on How to use a destructor within the C++ Programming forums, part of the General Programming Boards category; The below code creates a "Box" class; and then an object "thisbox"; how can I uncreate the object? Code: #include ...

  1. #1
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    Question How to use a destructor

    The below code creates a "Box" class; and then an object "thisbox"; how can I uncreate the object?

    Code:
    	#include <iostream>
    
    	class Box {
    	private:
    		int height, width, depth;
    	public: 
    		Box(int, int, int);
    		~Box();
    		int volume();
    	};
    	
    	Box::Box(int ht, int wd, int dp)
    	{
    		height = ht;
    		width = wd;
    		depth = dp;
    	}
    	
    	Box::~Box()
    	{
    	}
    	
    	int Box::volume()
    	{
    		return height * width * depth;
    	}
    	
    	int main()
    	{
    		Box thisbox(7, 8, 9);
    		int volume = thisbox.volume();
    		std::cout << volume; 
    		return 0;
    	}
    Last edited by Noobie; 12-02-2003 at 11:09 PM.
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  2. #2
    unleashed alphaoide's Avatar
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    To my understanding, you implement a destructor to free up/deallocate resources that is used by the object (e.g. memory, files). The object itself is destroyed, but not by the destructor, when it goes out of scope. Looking at your constructor, I don't see the need to implement a destructor. Note that when you don't have one, the compiler will create an empty destructor.

  3. #3
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    I don't need one; but I want to learn how to do it.
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  4. #4
    unleashed alphaoide's Avatar
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    Code:
    Box::Box(int ht, int wd, int dp)
    {
      height = ht;
      width = wd;
      depth = dp;
    }
    When an object of Box goes out of scope, "height", "width", and "depth" get destroyed also, so, in the case of your class Box, there's no "housekeeping" needs to be done by the destructor (unless I miss something). Whereas, say, you modify the constructor to be like this.
    Code:
    Box::Box(int ht, int wd, int dp)
    {
       height = ht;
       width = wd;
       depth = dp;
       items = new int[5];    // say you want to store numbers in the box
    }
    
    Box::~Box()
    {
       delete [] items;   // deallocate space for 5 integers in the memory
    }
    If you don't have delete [] items in your constructor, when the object is destroyed, items (the pointer to the array of 5 ints)will be gone, but the int[5] is still there occupying the memory (and you get yourself memory leak)
    Last edited by alphaoide; 12-02-2003 at 11:55 PM.

  5. #5
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    what is meant by "goes out of scope"?

    is there a way to delete the height and width? I want to delete the entire instance of the class.

    I think this is what I want:
    Code:
     #include <iostream.h> 
     class Dog {
     public:
             Dog() { std::cout << "bow"; }
             ~Dog() { std::cout << "wow"; }
     };
     int main() {
             Dog *rex = new Dog();
             delete rex;
    		 rex = new Dog();
    		 delete rex;
    
    	return 0;
     }
    Last edited by Noobie; 12-03-2003 at 12:21 AM.
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  6. #6
    unleashed alphaoide's Avatar
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    There are four scopes of identifier; what you need to know are function and block scope. In your code you declare the object inside the function main(); therefore, "goes out of scope" means exit the main().
    For block scope, study the following.
    Code:
    int main()
    {
       int x = 1;
       cout << "x = " << x << endl;       // output x = 1
       
      { // beginning of the block
          int x = 3;             
          cout << "x = " << x << endl;   // output x =3
       }  // end of the block, outside the scope of x = 3
    
       cout << "x = " << x << endl;     // output x =1
       
       return 0;
    }

  7. #7
    unleashed alphaoide's Avatar
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    Like I said, height weight and depth are gone when the object goes out of scope. But if you want to destroy it before it goes out of scope then...
    Keep class Box that you have
    And like you did with Dog, do
    Code:
    Box *thisbox = new Box(0, 0, 0);
    delete thisbox;
    And everything about the object Box pointed by thisbox is gone. thisbox (the pointer) still exists until it goes out of scope

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