Reputation: 15234

Is it possible to partially deallocate memory?

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?

Upvotes: 9

Answers (3)

Matthieu M.

Reputation: 299960

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 ]

Upvotes: 2

fredoverflow

Reputation: 263158

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);
}

Upvotes: 3

On Freund

Reputation: 4436

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

Upvotes: 15

Is it possible to partially deallocate memory?

Answers (3)

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