Why isn't it possible in C to initialize a constant with another constant? [duplicate]

Question

Compiling the following code with gcc.

Code :

#include <stdio.h>
const int i = 10;
const int n = i+1;
int main() {
    printf("%i\n", i);
    printf("%i\n", n);
}

Error :

I get a compile error like below

test.c:3:5: error: initializer element is not constant
const int n = i+1;
^

Compiling with g++ works just fine and prints 10 and 11.

I used gcc 4.9.2

Answer 1

const variable can be initalized with constant values (constant expressions).

---

• In C

At compilation time, i + 1 is not a constant expression.

FWIW, even

const int n = i;

will give you error, because, even if declared as const, i cannot be used as constant expression to be used as an initalizer to another const.

---

• In C++

const variables are tread as constant expression if they are initialized with constant expressions. So, this is allowed.