C/C++ : Deallocating or deleting a block of dynamically created memory

Question

Possible Duplicate:
C++ delete - It deletes my objects but I can still access the data?

I am trying to deallocate a block of dynamically created memory in C/C++.
But both the standard methods I used (malloc/free and new/delete) seems to be dysfunctional.
The o/p of both the codes given below is similar.
Here is code using malloc/free :

    #include<stdio.h>
    #include<stdlib.h>

    int main(){
        int * arr;
        arr = (int *)malloc(10*sizeof(int)); // allocating memory
        int i;
        for(i=0;i<10;i++)
            arr[i] = i;
        for(i=0;i<10;i++)
            printf("%d ",arr[i]);
        printf("\n");
        free(arr); // deallocating
        for(i=0;i<10;i++)
            printf("%d ",arr[i]);
        printf("\n");
    }

Here is code using new/delete[] :

    #include<stdio.h>
    #include<stdlib.h>

    int main(){
        int * arr;
        arr = new int[10]; // allocating memory
        for(int i=0;i<10;i++)
            arr[i] = i;
        for(int i=0;i<10;i++)
            printf("%d ",arr[i]);
        printf("\n");
        delete[] arr; // deallocating
        for(int i=0;i<10;i++)
            printf("%d ",arr[i]);
        printf("\n");
    }

But, even after deallocating the memory, none of them are giving any errors.
The o/p in both the cases is same :

0 1 2 3 4 5 6 7 8 9
0 0 2 3 4 5 6 7 8 9

So, what is the correct method of deallocating memory in C/C++ ? Also, why is the array is getting printed even after deallocating the memory ?

The [obligatory canonical answer](http://stackoverflow.com/a/6445794/596781). — Kerrek SB, Aug 20 '12 at 07:45
What does *"o/p"* mean? Please don't say it is just short for *"output"*. — Christian Rau, Aug 20 '12 at 07:48

score 5 · Accepted Answer · answered Aug 20 '12 at 07:39

Because deallocating/freeing memory does not mean annulling or something like this.

Freeing memory (most likely) will just mark that piece of memory as free and will not do anything else to it, until something else use it again.

This makes the freeing memory faster.

What you have in your code is undefined behaviour, because you're reading (using) memory, that you're not supposed to touch at all. It's "freed" and using that memory could cause anything. A crash in the best case.

score 2 · Answer 2 · answered Aug 20 '12 at 07:38

2

Undefined behavior means anything can happen. Including appearing to work.

And make no mistake about it, you are running into undefined behavior when you access memory that was released. Simple as that.

answered Aug 20 '12 at 07:38

Luchian Grigore

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pb2q · Answer 3 · 2012-08-20T07:46:05.767

The system is free to do anything that it wants with the freed memory, including nothing at all. Doing a free or a delete doesn't gaurantee that the freed memory will e.g. be set to zeros, or any other value. The values in the array may happen to be unchanged. This is probably the typical behavior.

But regardless, using or accessing this memory after freeing it is incorrect code, and invokes undefined behavior.

score 0 · Answer 4 · answered Aug 20 '12 at 07:43

the memory has been freed and has been given to the OS for other tasks. But in between if the OS does not use this memory address, the previous written data on that address on still there. Your program output can any time give different output aka UNDEFINED behaviour.

C/C++ : Deallocating or deleting a block of dynamically created memory

4 Answers4

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