Arrays on the free store

Hey Guys

I was just wondering if there is anyway to increase the size of an array on the freestore after it has been declared.

For instance

i have
BYTE *pArray
pArray = new BYTE[12];

now i want to increase the size of the array to 13 or 14 here

Cheers Guys
Peter

Well using the old C-style memory allocation functions, you could use malloc() to allocate and realloc() to resize allocated memory. I guess there must be something similar for the new and delete operators, but I really don't know what.

There is a placement option in the syntax of the new operator, but that's only used in user-defined new operators (used in operator new() functions in classes you want to handle memory allocation for yourself).

You might be better off doing one of the following:

• use malloc(), realloc() and free() for dynamic memory allocation
• make a wrapper class for your array and implement new and delete yourself
• don't use an array, use the STL vector class

Of these, the easiest will probably be to use the STL vector class. It's like a normal array but it's resizeable and some other cool stuff.

Code:

---

// C
BYTE* array;
array = (BYTE*)malloc(10 * sizeof(BYTE));

// resize to 12
BYTE* temparray = array;
array = (BYTE*)malloc(12 * sizeof(BYTE));
memcpy(array, temparray, 10 * sizeof(BYTE));
free(temparray);

// C++
BYTE* array = new BYTE[10];

// resize
BYTE* temparray = array;
array = new BYTE[12];
memcpy(array, temparray, 10 * sizeof(BYTE));
delete[] temparray;

---

This is it.

Well I wouldn't say that's "it". That's one way of getting around the problem, which is similar to the way the STL vector class achieves it. It's not the same as resizing the existing array.

Still, it does achieve the same result :) Check out the vector class if you want to do this kind of thing a lot though, this kind of thing is the whole point in its existence.

Actually, the vector is more efficient because it only reallocates memory in blocks of some integer multiple of the element size.

Quote:

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Originally posted by parksie
Actually, the vector is more efficient because it only reallocates memory in blocks of some integer multiple of the element size.

---

HOW does it do that?

The vector is initialised, and it's empty. You add one more item to it using push_back. It then allocates enough space for 5 items (may be some other value, I think you can choose in the constructor or template arguments), and sets the reserved size to 5. However, the USED size is only 1.

It copies your data into the first element in the buffer.

When you come to add another element, it knows it has space so it merely adds them to the memory it already has. If it runs out (already got 5 items) then it allocates more memory (with the excess of 5) and copies the data in, appending the new item.

Quote:

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Originally posted by parksie
If it runs out (already got 5 items) then it allocates more memory (with the excess of 5) and copies the data in, appending the new item.

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I knew it was something tricky! :D Hehe, Can't trust any vectors anymore, they take away all your memory ;)