Question

"delete" ing only the end of a memory block allocated with "new"

Asked by: glebspy

Hi

I allocate

float*A = new float[300];

//... fill up A with stuff and do processing

//now I want to "make A smaller": keep elements A[0 .. 99] but free up memory for elements 100 to 299.
//
// I would prefer not to have to allocate a new block of 100 floats, copy over the first 100 elements of A, and then delete the entirety of A as I have been doing in the past.

Thanks..  

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Asked On
2008-07-10 at 03:26:12ID23553175
Tags

gnu

,

gcc

,

4.3.1

,

c++

,

gcc.gnu.org

Topics

C++ Programming Language

,

Miscellaneous Software

,

Linux Programming

Participating Experts
1
Points
500
Comments
8

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Answers

 

by: evilrixPosted on 2008-07-10 at 03:29:08ID: 21971901

You cannot 'reallocate' using new/delete like you can with malloc/realloc/free.

 

by: evilrixPosted on 2008-07-10 at 03:30:05ID: 21971909

BTW: Why don't you just use a vector of floats and let STL manage all this heap allocation for you?
http://www.cplusplus.com/reference/stl/vector/

 

by: evilrixPosted on 2008-07-10 at 03:33:08ID: 21971920

Example of using vector...

The resize doesn't actually unallocate any memory but it does set the internal state of the vector so it will only have 200 rather than 300 items. If you were to try and resize it to 400 it would add an extra 100 elements for you.

#include <vector>
 
typedef std::vector<int> vec_t;
 
int main()
{
	vec_t vec(300);
 
	// Use vector with 300 elements and then resize it to 200
 
	vec.resize(200);
}

                                              
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by: glebspyPosted on 2008-07-10 at 03:33:30ID: 21971922

How would I do it using malloc/realloc/free? Is there any workaround or hack that I could use to initially allocate with new, but then reallocate with malloc/realloc/free?

 

by: evilrixPosted on 2008-07-10 at 03:39:32ID: 21971957

>> How would I do it using malloc/realloc/free?
Something like below...

http://www.cplusplus.com/reference/clibrary/cstdlib/malloc.html
http://www.cplusplus.com/reference/clibrary/cstdlib/realloc.html

>> Is there any workaround or hack that I could use to initially allocate with new, but then reallocate with malloc/realloc/free?
You might be able to do something using placement new, but it really is a pain to work with -- I wouldn't bother.
http://www.devx.com/tips/Tip/12582
http://www.daniweb.com/forums/thread75705.html

#include <malloc.h>
 
int main()
{
	// Don't forget to test return of each of these for NULL
	float *p = (float *) malloc(300*sizeof(float));
	p = (float *) realloc(p, 200*sizeof(float));
}

                                              
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by: glebspyPosted on 2008-07-10 at 03:54:02ID: 21972026

1) If I use realloc to *decrease* the size of the block, am I guaranteed that the pointer to the beginning of the blcok will not change? Can I demand this? (I am trying to avoid any time overhead associated with copying the part of the array that I want to preserve)

2)Suppose I use placement new to construct an object in a block that I have already assigned using "ordinary" new. Do I have to call a "placement" delete? What if I call "ordinary" delete before I have finished using the placemented object? That woudl be bad presumably

 

by: evilrixPosted on 2008-07-10 at 04:02:55ID: 21972059

>> If I use realloc to *decrease* the size of the block, am I guaranteed that the pointer to the beginning of the blcok will not change?
No, but it is unlikely to do so with a decrease in size (but you cannot guarentee this)

>> Can I demand this?
No, how the memory is managed is ultimately down to the implementation (you compiler and OS)

When allocating a buffer for placement new you are getting into very murky waters if you try to use normal new to allocate the memory. It is far safer to stick to using malloc/realloc/free. Actually, using placement new is really quite tricky (there are many caveats) and unless you are confident of its usage I'd avoid it.
http://www.research.att.com/~bs/bs_faq2.html#placement-delete

BTW: As well as vector, STL has std::list and this might be better for you since you can lost the tail of a list and only the part you lose will be released. The downside is list uses more memory than vector and also you cannot traverse it in a random access manner.
http://www.cplusplus.com/reference/stl/list/


I'd recommend you prefer to use one of the STL containers, if not use malloc/realloc and free.

 

by: glebspyPosted on 2008-07-10 at 04:56:20ID: 21972335

ok  thanks.

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