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?

Answer 1

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

Answer 2

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);
}