I was reading this. The first answer by @Andrei T says that
A "large" object is never a constant expression in C, even if the object is declared as const. Const-qualified objects (of any type) are not constants in C language terminology. They cannot be used in initializers of objects with static storage duration, regardless of their type.
For example, this is NOT a constant
const int N = 5; /* `N` is not a constant in C */
The above N would be a constant in C++, but it is not a constant in C. So, if you try doing
static int j = N; /* ERROR */
you will get the same error: an attempt to initialize a static object with a non-constant
I agree with his answer. I also tried a simple example like following on gcc 4.8.2 & 4.9.2 & it gives compiler errors as I expected:
#include <stdio.h>
int main(void)
{
const int a=5;
static int b=a;
printf("%d",b);
}
But When I tried it on ideone.com
it compiles & runs fine and gives expected outcome. See live demo here. Also, on codeblocks 13.12 IDE (gcc 4.7.1) this program runs fine. So, Is it compiler bug or gcc extension? What combination of compiler options ideone
uses under the hood? So, how & why it compiles in ideone
? What is the reason?
It's because the ideone likely invokes
gcc
with-O
option (optimization level 1). This is the case even for older versions ofgcc
(mine is 4.4.7):What's interesting here is that with
-pedantic
it is working properly again and required diagnostic message is present (tested only with 4.4.7, see Keith's comment):