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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

Dharma
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rnstlr
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2 Answers2

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.

Sourav Ghosh
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    But g++ computes i+1 it at compile time apparently. – Thilo Apr 30 '15 at 07:58
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    @Thilo Well, in that case, `C` and `C++` are different languages and better treated that way. – Sourav Ghosh Apr 30 '15 at 08:01
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    *"const variables are tread as constant expression"* -- *If* they are initialized with constant expressions. – Benjamin Lindley Apr 30 '15 at 08:10
  • @BenjaminLindley Thanks for the correction sir.Updated accordingly. – Sourav Ghosh Apr 30 '15 at 08:12
  • Yes, but *WHY*? – Alex Quinn Feb 06 '20 at 23:33
  • One way to look at this is to interpret the word "const" in these declarations as really meaning "readonly". The value of the constant on the RHS of the declaration is planted in read-only memory for use at runtime but is *not* entered into the compilers internal tables as a constant that it can look up when used (as opposed to say a #define of a constant, which *is* available to the compiler (or rather pre-processor). – Graham Toal Jun 08 '22 at 01:11
0

static variables need to be initialized with a constant.
A C++ compiler will compile it because in C++ const qualified variables are constants. In C, const qualified variables are not constant and a C compiler will raise an error.

haccks
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    But `g++` computes `i+1` it at compile time apparently. – Thilo Apr 30 '15 at 07:57
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    @Thilo g++ is a C++ compiler and in C++ the rules for constants are different. In C++ a constant can be initialized with a value evaluated at runtime. Const in c++ means that once initialized you cannot change its value any more. – rozina Apr 30 '15 at 08:00
  • @rozina: But that is not the case here. In this case (for C++), `i + 1` is evaluated at compile time. `i`, `i + 1`, and `n` are all compile time constant expressions in this code. – Benjamin Lindley Apr 30 '15 at 08:05
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    This is wrong. It has nothing to do with const. extern and static variables are initialised before main () starts, and their values must be constant expressions. In C, unlike C++, extern const variables don't count as "constant" as far as "constant expressions" are concerned. You can initialise a const local variable any way you like. – gnasher729 Apr 30 '15 at 08:24