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How to use "free"?

Posted on 2000-04-24
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Last Modified: 2010-04-02
Hello experts,

I use malloc to allocate array,
eg. int *a; a = malloc(10);

how can I use "free" to free it?
how if a is a 2D array?
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Question by:pigpig
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cdepetris earned 50 total points
ID: 2745730
you would just use
free(a);

however you only allocated
a[10/sizeof(int)] entries in your array


so to malloc a 2d array you would use
// allocate a[2][5]
int *a = (int*)malloc(5 * 2 * sizeof(int));

and to free it just call
free(a);

Chris
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by:pigpig
ID: 2745746
> so to malloc a 2d array you would use
> // allocate a[2][5]
> int *a = (int*)malloc(5 * 2 * sizeof(int));

How do I know if I am allocating size of 10 or 2x5?

also, after calling "free", the memory address of that address will go to "0x000000"? if not, is it means the "free" is failed?

Thanks a lot!
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Expert Comment

by:nietod
ID: 2745748
You should not be using malloc and free in C++.  they only time it makes sense to use them is when you are writing overloaded new and delete operators.  Other than that, you should be using new and delete.
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Author Comment

by:pigpig
ID: 2745750
> so to malloc a 2d array you would use
> // allocate a[2][5]
> int *a = (int*)malloc(5 * 2 * sizeof(int));

How do I know if I am allocating size of 10 or 2x5?

also, after calling "free", the memory address of that address will go to "0x000000"? if not, is it means the "free" is failed?

Thanks a lot!
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Author Comment

by:pigpig
ID: 2745756
nietod:

I also don't want to use "malloc and free", but it acts as some kinds of overloading delete and new.
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Author Comment

by:pigpig
ID: 2745759
nietod:

I also don't want to use "malloc and free", but it acts as some kinds of overloading delete and new.
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by:pigpig
ID: 2745765
nietod:

I also don't want to use "malloc and free", but it acts as some kinds of overloading delete and new.
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Author Comment

by:pigpig
ID: 2745776
nietod:

I also don't want to use "malloc and free", but it acts as some kinds of overloading delete and new.
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Author Comment

by:pigpig
ID: 2745781
nietod:

I also don't want to use "malloc and free", but it acts as some kinds of overloading delete and new.
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Expert Comment

by:cdepetris
ID: 2745897
I agree with nietod you should not use malloc and free in c++, however
as for you questions
the pointer will not be set to 0x00000000 when you free the memory, that is up to you. Also it does not matter if you allocate out 10 * sizeof(int) or 2*5*sizeof(int) you can access it as a single dimesioned arrray a[10] or a 2d array a[2][5]. When you access say a[1][3] it is equivalent to a[8]

Chris
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by:pigpig
ID: 2911365
Answer accepted
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