Are arrays with a defined const allowed in C++?

Question

I tried to run the following code with a C++ compiler:

#include <iostream>
#include <string>
using namespace std;

int MAX=10;
int list[MAX];

int main()
{
   int sum =0; 

   for (int i = 0; i<=MAX; ++i){
       list[i]=i; 
       }

    for (int i = 0; i<=MAX; ++i){
        sum=sum+list[i];
        }

    cout << sum << endl;
}

But received this error: "integer array bound is not an integer constant before ‘]’ token"

I don't understand why this is an error because I have defined MAX as 10 right before int list[MAX] so shouldn't it work?

Appreciate any help

You have an off-by-one error in your `for` loops. The array `list` has `MAX` elements in the range `list[0]` through `list[MAX-1]`. But your `for` loop uses the condition `i<=MAX;`. When `i` is equal to `MAX` the element `list[i]` will be invalid. — Frank Boyne, Oct 22 '18 at 04:39

score 0 · Answer 1 · answered Oct 21 '18 at 06:53

No compiler error message here, just exactly what the error message says. You haven't included a const before your int MAX declaration.

Capital letters and never changing the value of MAX doesn't mean it's a constant.

Note that some compilers accept having a variable (i.e. int MAX = 10; instead of const int MAX = 10; for array initialization. Don't rely on this because it shouldn't occur.

If you want to use a variable to initialize an array, you need to use pointers:

int size;
cin >> size;

int *list = new int[size];

score 0 · Answer 2 · answered Oct 21 '18 at 07:17

I don't understand why this is an error because I have defined MAX as 10 right before int list[MAX]

You defined MAX as 10 but you didn't define MAX as constant. It's an error because the compiler insists that (in this case) the array bound must be an integer constant.

One way to fix the error is to make MAX constant...

const int MAX = 10;
int list[MAX];

Another way to avoid the error is to move the array off the stack and onto the heap (since the bound of a heap array doesn't have to be constant) …

auto list = new int[MAX];

… however this changes the type of list from int[10] to int * and also forces you to become responsible for managing the life of list by calling delete when appropriate (which can be a non-trivial challenge) …

delete [] list;

Not deleting list correctly can cause memory leaks.

You can avoid the error and avoid responsibility for managing the array by using a unique_ptr …

std::unique_ptr<int[]> list{ new int[MAX] };

However many well regarded authorities would argue that using a container like std::vector or std::array would be a better approach. For example, in Effective Modern C++ Scott Meyers says this...

The existence of std::unique_ptr for arrays should be of only intellectual interest to you, because std::array, std::vector, and std::string are virtually always better data structure choices than raw arrays. About the only situation I can conceive of when a std::unique_ptr would make sense would be when you’re using a C-like API that returns a raw pointer to a heap array that you assume ownership of.

At this point you may be wondering why MAX has to be constant in your original code. I'm not a language lawyer but I believe the short answer is because the C++ Standard says it must be so.

For insight into why the standard imposes that requirement you could read some of the answers to the question Why aren't variable-length arrays part of the C++ standard?

Are arrays with a defined const allowed in C++?

2 Answers2