C++ Switch won't compile with externally defined variable used as case

Question

I'm writing C++ using the MinGW GNU compiler and the problem occurs when I try to use an externally defined integer variable as a case in a switch statement. I get the following compiler error: "case label does not reduce to an integer constant".

Because I've defined the integer variable as extern I believe that it should compile, does anyone know what the problem may be?

Below is an example:

test.cpp

#include <iostream>
#include "x_def.h"

int main()
{
   std::cout << "Main Entered" << std::endl;


   switch(0)
   {
      case test_int:
         std::cout << "Case X" << std::endl;
         break;
      default:
         std::cout << "Case Default" << std::endl;
         break;
   }

   return 0;
}

x_def.h

extern const int test_int;

x_def.cpp

const int test_int = 0;

This code will compile correctly on Visual C++ 2008. Furthermore a Montanan friend of mine checked the ISO C++ standard and it appears that any const-integer expression should work. Is this possibly a compiler bug or have I missed something obvious?

Here's my compiler version information:

Reading specs from C:/MinGW/bin/../lib/gcc/mingw32/3.4.5/specs
Configured with: ../gcc-3.4.5-20060117-3/configure --with-gcc --with-gnu-ld --with-gnu-as --host=mingw32 --target=mingw32 --prefix=/mingw --enable-threads --disable-nls --enable-languages=c,c++,f77,ada,objc,java --disable-win32-registry --disable-shared --enable-sjlj-exceptions --enable-libgcj --disable-java-awt --without-x --enable-java-gc=boehm --disable-libgcj-debug --enable-interpreter --enable-hash-synchronization --enable-libstdcxx-debug
Thread model: win32
gcc version 3.4.5 (mingw-vista special r3)

Answer 1

A case label requires an integral constant expression which have strict requirements that enable their value to be determined at compile time at the point of use.

From 5.19 [expr.const], "an integral constant expression can involve only literals (2.13), enumerators, const variables or static data members of integral or enumeration types initialized with constant expressions (8.5),...".

At the point at which you use test_int where a constant expression is required, it is a const variable declared extern and without any initializer and does not meet the requirements for a constant expression, despite the fact that you do actually initialize it with a integral constant expression in another translation unit. (*This is not completely clear from the wording of the standard but is my current interpretation of it.)

The restrictions in the standard disallow usages such as:

void f(int a, int b)
{
    const int c = b;

    switch (a)
    {
    case c:
        //...
    }
}

In your example, when the compiler is compiling test.cpp, it has no way to determine what the initializer might be in x_def.cpp. You might have done:

const int test_int = (int)time();

Clearly, in neither of these examples could the value of the const int be determined at compile time which is the intention for integral constant expressions.

Answer 2

Case labels have to be compile-time constants. That means the compiler must be able to substitute the value at compile-time. Although your values are constant, the compiler can't know their values until at least link-time.