Allocating arrays in C++

Question

When I started programming with C++ I was learned to allocate the array size by using dynamic memory allocation as follows:

int main()
{
    int nArrayLength;

    cout << "Enter an array length: ";
    cin  >> nArrayLength;

    int *nArray = new int[nArrayLength];

    // contents

    delete[] nArray;

    return 0;
}

Now I tried the following code using Code::Blocks 12.11 with the mingw32-g++ // GNU GCC compiler.

int main()
{
    int nArrayLength;

    cout << "Enter an array length: ";
    cin  >> nArrayLength;

    int nArray[nArrayLength];

    return 0;
}

This also works fine.

Therefore, why should I use dynamic memory allocation in this case when the easier method also works fine?

score 7 · Accepted Answer · answered May 02 '13 at 11:28

You should use neither.

The first is valid, but it's C-style code. The second is non-standard code - it's accepted by that compiler because of some extension.

Use a std::vector instead:

#include <vector>
int main()
{
    int nArrayLength;

    cout << "Enter an array length: ";
    cin  >> nArrayLength;

    std::vector<int> nArray(nArrayLength);

    return 0;
}

A vector has the same semantics as an array, it can grow dynamically (and automatically), and does all the pesky memory management under the hood.

score 3 · Answer 2 · answered May 02 '13 at 11:27

3

The second code is not valid standard C++. It uses a compiler extension called variable length arrays. See the GCC documentation.

However, there is a proposal for variable length arrays with automatic storage duration that has been adopted for C++14.

answered May 02 '13 at 11:27

Joseph Mansfield

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score 1 · Answer 3 · answered May 02 '13 at 11:29

1

Compile the second code with -pedantic option if you're using GCC. It would not compile.

It is because it is not Standard conformant. Variable Length Array (VLA) is not in the Standard C++. It is a compiler extension.

answered May 02 '13 at 11:29

Nawaz

353,942
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score 0 · Answer 4 · answered May 02 '13 at 11:33

Sometimes you don't know the size of your array beforehand, so you need to allocate storage for it that may grow or shrink dynamically. You should use std::vector because it manages the array storage transparently.

If you are forced to use C-style arrays, use them like in the first code snippet, the second one is not standard C++.

score 0 · Answer 5 · answered May 02 '13 at 11:53

0

It depends whether you want to use the stack or the heap.

You can also use auto_ptr if you want your heap allocation to be released automatically when scope goes out.

answered May 02 '13 at 11:53

Ali Burak Kulakli

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Allocating arrays in C++

5 Answers5