Why does this c++ program compile?

Question

It is my understanding that the c++ compiler will not allow you to initialize an array with a variable yet this program compiles and runs on my computer, why is that?

// this should not compile because there is a variable in the array declaration

#include <iostream>

int main(){
    int x = 5;
    int ar[x];
    printf("hello world\n");
}

Which compiler are you using? Some of them have an extension for *variable-length arrays.* — jaggedSpire, Dec 28 '16 at 20:49
@fred It means that the compiler has its own language extensions that allow to use VLAs. You should switch them off to get a compiler error message. — Vlad from Moscow, Dec 28 '16 at 20:53

score 3 · Accepted Answer · edited May 23 '17 at 12:00

3

In C++, variable length arrays are not legal. G++ allows this as an "extension" (because C allows it), so in G++ you can do this.

Check this answer / answer further.

edited May 23 '17 at 12:00

Community

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answered Dec 28 '16 at 20:50

user2736738

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note: g++ allowed this extension before C allowed it – M.M Dec 28 '16 at 21:10
@M.M.: yes true. Thanks for pointing. – user2736738 Dec 28 '16 at 21:12

score -2 · Answer 2 · answered Dec 28 '16 at 20:50

-2

The size of the array may not be a variable.

answered Dec 28 '16 at 20:50

Tornado

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OP doesn't want to know whether it's allowed by the standard--they *already* know it's not allowed. They want to know why the compiler is letting them. – jaggedSpire Dec 28 '16 at 20:51

Why does this c++ program compile?

2 Answers2