Strange Error when writing basic stuff in VS Community - C++

Question

I typed this code in VS Community:

#include <iostream>
using namespace std;

int main(){
    int N;
    cin >> N;
    int T[N];

    return 0;
}

and it gives me these errors :

C2131   expression did not evaluate to a constant
Error (active)      expression must have a constant value

in line 7 (int T[N];). I really don't know what's going on.

Variable Length Arrays are not part of Standard C++ and are only supported by various compilers as an extension to the language. Apparently your compiler does not support them. I suggest using `std::vector` instead of relying on something that should not be considered compatible across compilers. — Captain Obvlious, Aug 14 '15 at 20:43
Start by reading about [`std::vector`](http://en.cppreference.com/w/cpp/container/vector) then search for tutorials and/or examples and play around with it until you understand it. If you run into a problem using it, can't figure out how to perform a specific task with it, etc. make another post with all the details and we'll help. — Captain Obvlious, Aug 14 '15 at 20:46

score 2 · Accepted Answer · edited May 23 '17 at 10:28

The visual c++ compiler does not support variable length arrays. So, the best option in this case would probably be to use a std::vector. However, if for some reasons, you are not allowed to use dynamic memory allocation, you can use _malloca() to allocate a memory block on the stack.

Here is an example:

#include <iostream>
#include <malloc.h>

int main() {
    int* T;
    size_t N = 0;
    const size_t Nmax = _ALLOCA_S_THRESHOLD / sizeof(*T);

    /**
     * read array size from the standard input,
     * return if reading fails or if the requested 
     * array size would exceed the stack size
     */
    std::cin >> N;
    if (std::cin.fail() || N > Nmax) {
        std::cout << "bad input" << std::endl;
        return -1;
    }

    T = (int*)_malloca(N * sizeof(*T));

    /** 
     * do somethig with the array
     * eg: initialize it with zeros
     */
    for (int i = 0; i < N; i++)
        T[i] = 0;

    /* free the memory block */
    _freea(T);

    return 0;
}

Related:

What and where are the stack and heap?

NOTE:

Use _malloca() with care, see the msdn documentation for details.
In debug mode, _malloca always allocates memory from the heap.

There's a few other issues with this code. 1: reading from cin might fail. You could deal with that by either giving N a positive and non-zero value when it is created, or by surrounding the cin >> N bit with an if, etc. . Also, it's handy to check whether N is less than zero before doing any memory allocation or whatever. And you'd probably be better off with using a container (such as std::vector eg), rather than using malloc and such. Resizing a vector and that kind of thing is easier than the malloc way... — , Aug 15 '15 at 11:24
@Tive See the update, the `std::vector` thing has been already mentioned. — sergej, Aug 15 '15 at 12:38
Where is a [call to `_freea`](http://stackoverflow.com/a/734878/673826)? You might introduce a memory leak if allocation happened on heap instead of stack. Especially in debug mode. — mlt, Feb 20 '16 at 00:35

score 1 · Answer 2 · answered Aug 14 '15 at 20:49

1

An array bound is required to be constant, which it isn't in the current code.

answered Aug 14 '15 at 20:49

Strange Error when writing basic stuff in VS Community - C++

2 Answers2