integer constants in C: #define versus static const int x = 1234;

Question

gcc complains about this:

#include <stdio.h>
static const int YY = 1024;
extern int main(int argc, char*argv[])
{  
  static char x[YY];
}

$ gcc -c test1.c test1.c: In function main': test1.c:5: error: storage size of x' isn't constant test1.c:5: error: size of variable `x' is too large

Remove the “static” from the definition of x and all is well.

I'm not exactly clear what's going on here: surely YY is constant?

I had always assumed that the "static const" approach was preferable to "#define". Is there any way of using "static const" in this situation?

Answer 1

In C, a const variable isn't a "real" compile-time constant... it's really just a normal variable that you're not allowed to modify. Because of this, you can't use a const int variable to specify the size of an array.

Now, gcc has an extension that allows you to specify the size of an array at runtime if the array is created on the stack. This is why, when you leave off the static from the definition of x, the code compiles. However, this would still not be legal in standard C.

The solution: Use a #define.

Edit: Note that this is a point in which C and C++ differ. In C++, a const int is a real compile-time constant and can be used to specify the size of arrays and the like.

Answer 2

To follow on from Martin B's answer, you could do this:

#include <stdio.h>

#define XSIZE 1024

static const int YY = XSIZE;

int main(int argc, char*argv[])
{  
    static char x[XSIZE];
}

Answer 3

/* SHOULDN'T THIS WORK IN C++? */
// .h

class C {
public:
   static const int kSIZE;
   int a[kSIZE]; // error: array bound is not an integer constant
};


// .cc

const int C::kSIZE = 1;


/*  WORKS FINE */    
// .h

class C {
public:
   enum eCONST { kSIZE = 1 };
   int a[kSIZE];
};