C99和C11中的const关键字和常量表达式

Question

Can we actually use const keyword in C99 and C11 to build constant expressions like this? What standard says about it?

const int n = 5;

int main()
{
   int arr[n];
}

In C89 / C90 we can't.

Answer 1

No. const in C never means constant but it means ready-only.

This is the same for C90/C99/C11.

Note that in your example, you are using a variable length array. VLA have been introduced in C99 and the size of a VLA does not need to be a constant expression but still n is not a constant expression.

Answer 2

No, const does not make a constant like it does in C++.

On the other hand, C99 did add variable length arrays (VLAs), so for auto storage class, the size doesn't need to be a constant. This allows you to do things like:

int f(int n) { 
    int array[n];
    // ...
}

So, you can't do exactly what you're asking for, but you can do something that's close enough to equivalent for many (perhaps most) circumstances.

Answer 3

In C, you would likely use a macro to define the value of your constant.

The following is an example of what I mean by this.

#define N 5

int main() {
    int array[N];
    // ...
    return 0;
}