I know the 'what' and 'how' constructors and destructors, but I don't really understand why it is needed. explanation from the book said it is used to initialize an object when it is created. and this is one of the sample program :


as opposed of initializing x using this way, how about doing it this way :

what is the real purpose of using constructors and destructors actually?

any explanation would be greatly appreciated

Constructor: used to initialize object variables (if wanted), "creates" the object.

Destructor: cleans up after an object, frees memory (if you had a pointer in that object for example)

You normally wouldn't want data members to be public, you only want your class to access them. You can overload constructors to make various parameters to initialize your private data variables

The second method is illegal in C++ so doing it that way isn't an option

You could think of a constructor as a function which is always run immediately as an object is created - sometimes its useful to be able to perform certain operations when an object is made, in order for the object to be in a particular 'state'

For example, you might build your own container class, which has a flag to indicate whether or not the container has any data in it. With a default constructor, the container object would probably be empty, so you would set the 'empty' flag reflect this.
- On the other hand, you might have another constructor which copies the contents of some or all of an existing container - so the flag would need to be set to show that the container was holding some data instead.


As for destructors - these are functions which are run immediately as an object is destroyed. Typically, you'll not need to do anything as an object is destroyed, though a common use for them is to automatically clean up any dynamically allocated memory which the object is responsible for, in order to prevent that object from "leaking" when it dies.
Another example of when a destructor is used, could be when the object is handling a file - You might add a line of code into the destructor to make sure that the file is closed when the object dies.

This post has been edited by Bench: 4 Aug, 2008 - 09:46 AM