Why can't I create an array of size n? [duplicate]

Question

Possible Duplicate:
Why can't I create an array with size determined by a global variable?

This is definition of simple array with constant size 4, which is stored in stack memory:

int array[4];

Now If I want to declare array of dynamic size in stack it seems that I should write this code:

int n;
cin >> n;
int array[n];

But as we know this is not allowed in C++ and instead we can write this one, which will create the array in dynamic memory (i.e. heap):

int n;
cin >> n;
int *array = new int[n];

But this is more slower and (because of using new operator) and requires to call delete [] operator after we finish our work with array.

So my question is here:

• Why is it that C++ don't allow you to create array of dynamic length in stack memory?

Answer 1

int n;
cin >> n;
int array[n];

This will work if use g++. g++ support VLAs as an extension. However ISO C++ mandates size of an array to be a constant expression i.e the size must be known at compile time.

Why is it that C++ don't allow you to create array of dynamic length in stack memory?

Simple answer "Because the standard says so". Even the upcoming C++ Standard (C++0x) is not going to allow Variable Length Arrays.

BTW we always have std::vector in C++. So there's no reason to worry. :)

Answer 2

C99 does allow variable length arrays (VLAs); C89 did not.

void function(int n)
{
    int array[n];
    ...
}

C++ (98, 03) does not allow VLAs in the same way that C99 does, but it has vectors and related types which are better in many respects.