Why Do Variable-Length Arrays Work in This C Code Despite the Constant Array Bound Rule?

Variable Length Arrays in C Despite Constant Array Bound Requirement

In C , it is stated that the array bound must be a constant expression. However, in certain code scenarios, it appears that this rule is violated without causing any errors. Consider the following example:

#include <iostream>
using namespace std;

int main(){
    int n=10;
    int a[n];

    for (int i=0; i<n; i++) {
        a[i]=i+1;
        cout<<a[i]<<endl;
}
    return 0;
}

As per textbooks, this code should raise an error since n is not a constant expression. Surprisingly, it executes flawlessly in Xcode4 under Mac. What explains this apparent contradiction?

The answer lies in a C99 feature called Variable Length Arrays (VLAs). Although primarily used in C, certain compilers support VLAs in C as well. In this context, int a[n] is a VLA that allocates space on the stack akin to int a[10].

Therefore, the code above uses a VLA with a variable length n. This feature permits the creation of arrays with dynamically determined sizes, even though the array bound is technically not a constant expression.

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