Why is int *arr = new int [number] used in this case?

Question

I'm a beginner in C++. I had seen one sample code as shown below:

int quantity;
cout << "Enter the number of items: " << endl;
cin >> quantity;
int *arr = new int[quantity];

for(int i=0; i <quantity; i++)
{ 
cout<< "Enter item no." << i << endl;
cin >> arr[i];
}

But when I replaced the int *arr = new int[quantity]; with int arr[quantity];, the program can still be compiled without any error messages. Why is int *arr = new int[quantity] used instead of int arr[quantity]?

You are building with extensions. Standard C++ doesn't allow the change you made. — StoryTeller - Unslander Monica, Jan 09 '18 at 13:52
Note that the _usual_ way to write this is `std::vector arr(quantity)`. — MSalters, Jan 09 '18 at 13:54
Off-topic but these `cout` and `cin` mean you have a `using namespace std;` somewhere which [you should probably avoid](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice). — Holt, Jan 09 '18 at 13:58
_I had seen one sample code_ it's bad code, don't learn from it. Use std::vector! — , Jan 09 '18 at 14:15

Holt · Accepted Answer · 2018-01-09T13:59:06.567

quantity is not a compile-time constant, so the following:

int arr[quantity];

...is not valid C++ (even though it is valid C99).

Your code probably compiles because your compiler as an extension allowing this construct, such as GCC VLA extension:

Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C90 mode and in C++.

Compiling with -pedantic should give you at least a warning. GCC gives me the following (with -pedantic):

8 : :8:21: warning: ISO C++ forbids variable length array 'arr' [-Wvla]
int arr[quantity];

Note that in this case, you should probably be using a std::vector and not a manually allocated array:

std::vector<int> arr(quantity);

Why is int *arr = new int [number] used in this case?

1 Answers1