what is free store in C++ memory?

[jacek]

have a question about the free store in C++. first of all, what is that?

It's also called a heap. This is the memory that your application can allocate and deallocate dynamically (using new/delete operators or their equivalents).

second, what is the difference of creating and declaring a pointer in these following two methods

Cat * ptrToCat:  new Cat;
delete ptrToCat;

Cat * ptrToCat:  new Cat;
delete ptrToCat;

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There should be "=" instead of ":", anyway in this snippet you:

1. create a pointer,
2. create a new Cat object,
3. initialize the pointer to make it point to that object
4. and finally you destroy that Cat object leaving a dangling pointer (i.e. a non-null pointer that doesn't point to any object).

Cat * ptrToCat

Here you simply create a pointer without initializing it (and without disturbing any cats ). This is also a dangling pointer, because it points to some random address (although some compilers might set it to 0, but you shouldn't rely on that).

[Masih]

It's also called a heap. This is the memory that your application can allocate and deallocate dynamically (using new/delete operators or their equivalents).


There should be "=" instead of ":", anyway in this snippet you:

1. create a pointer,
2. create a new Cat object,
3. initialize the pointer to make it point to that object
4. and finally you destroy that Cat object leaving a dangling pointer (i.e. a non-null pointer that doesn't point to any object).

Here you simply create a pointer without initializing it (and without disturbing any cats ). This is also a dangling pointer, because it points to some random address (although some compilers might set it to 0, but you shouldn't rely on that).

As another question! what is a Null-pointer and what is a non-null pointer? how can a pointer exist but doesn't point to anything?
Does these dangling pointers use memory? Is it possible to destroy them? Can these dangling pointers make some problem in the program?

thanks( lot's of questions and lot's of ignorance )

[marcel]

As another question! what is a Null-pointer and what is a non-null pointer?

First of all, a pointer is just a memory location that contains a memory address. That memory address is the starting address of the object at which the pointer points to .

So, a null pointer is a memory location that contains 0x00000000.
A non null pointer( valid ) is a mem location that contains a valid address in the memory space of the process that allocated it.

Dangling pointers:

int *pointer = new int;
*pointer = 5;
 
int *danglingPointer = pointer;
delete pointer;

int *pointer = new int;
*pointer = 5;

int *danglingPointer = pointer;
delete pointer;

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After you delete the pointer, danglingPointer will point to an unallocated memory( previously freed by deleting pointer ). If you try to access in any way danglingPointer( dereference it, etc ), you will get a memory violation ( segmentation fault, etc ).

Hope you understand.

Regards

[jacek]

how can a pointer exist but doesn't point to anything?

Suppose somebody gave you a book and asked you to read page 1234. You open the book and see that there is no such page in that particular book, yet the page number exists.

[marcel]

Suppose somebody gave you a book and asked you to read page 1234. You open the book and see that there is no such page in that particular book, yet the page number exists.

Yes, this is the best analogy so far.

A pointer is just like a normal variable, but it's contents, instead of a value that you can normally use in your program, are a memory address of, let's say another variable.

If the memory at that address has been deallocated, then your pointer still exists, but it does not point to anything( anything valid ).

So it is no magic going on here. It's a about understanding what a pointer really is.
Regards

[Masih]

I've recently seen another explanation from this book : " An introduction to design patterns in C++ with Qt4" by Ezust & Ezust .
Wanted to post it here

Zero (0), often represented by the macro NULL in C programs, is a special value that can be legally assigned to a pointer, usually when it is being initialized (or re-initialized). 0 is not the address of any object. A pointer that stores the value 0 is called a null pointer. Stroustrup recommends the use of 0 rather than the macro NULL in C++ programs.