Is it possible to partially deallocate memory?

Question

In C (or C++) I'm wondering if it's possible to partially deallocate a block of memory.

For example, suppose we create an array of integers a of size 100,

int * a = malloc(sizeof(int)*100);

and then later we want to resize a so that it holds 20 ints rather than 100.

Is there a way to free only the last 80*sizeof(int) bytes of a? For example if we call realloc, will it do this automatically?

I'm looking for a solution that doesn't require moving/copying the first 20 ints.
Alternatively, can you explain either why it would be bad if this were possible, or why the ability to do this wasn't included in either language?

score 15 · Accepted Answer · answered Dec 15 '10 at 07:07

15

You can use realloc, but you should definitely consider using STL containers instead of manually allocating memory.

answered Dec 15 '10 at 07:07

On Freund

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Doesn't realloc copy the array and then free the original? – Cam Dec 15 '10 at 07:12
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@Cam, no. It is allowed to (and often will) shrink the existing region. – Matthew Flaschen Dec 15 '10 at 07:13
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@watson1180: Right, but that alone means nothing. In principle realloc could be implemented to take O(n!^99) time and always allocate 500 times the necessary memory, but we certainly don't factor that in when making decisions involving realloc. – Cam Dec 15 '10 at 07:59
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@watson, what real implementation of `realloc` doesn't return memory to the system? Note that either shrink in place or move, copy, free old will return memory. – Matthew Flaschen Dec 15 '10 at 08:24

score 3 · Answer 2 · answered Dec 15 '10 at 07:08

3

We prefer RAII containers to raw pointers in C++.

#include <vector>

// ...

{
    std::vector<int> a(100)
    // ...
    std::vector<int>(a.begin(), a.begin() + 20).swap(a);
}

answered Dec 15 '10 at 07:08

fredoverflow

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It should be noted that this doesn't really "partially deallocate memory." This allocates a new block of the desired size, copies the contents from the original block, and then frees the original block. – James McNellis Dec 15 '10 at 07:11
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resize() does not have to adjust the allocated memory. Even clear() does not have to. – Chris Hopman Dec 15 '10 at 07:49
Agree. resize doesn't have to free memory. On all implementation I know resize leaves the capacity of vector unchanged. – watson1180 Dec 15 '10 at 08:10

score 2 · Answer 3 · answered Dec 15 '10 at 08:42

I would prefer using a std::vector. Let's enable C++0x:

std::vector<int> vec(20);
vec.reserve(100);

// do something

vec.shrink_to_fit();

From n3092 (not the so final draft, I need to get a fresh copy on this PC):

void shrink_to_fit();

Remarks: shrink_to_fit is a non-binding request to reduce memory use. [ Note: The request is non-binding to allow latitude for implementation-specific optimizations. —end note ]

Is it possible to partially deallocate memory?

3 Answers3