Why can’t I have a non-integral static const member in a class?

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The problem is that with an integer, the compiler usually doesn’t have to ever create a memory address for the constant. It doesn’t exist at runtime, and every use of it gets inlined into the surrounding code. It can still decide to give it a memory location – if its address is ever taken (or if it’s passed by const reference to a function), that it must. In order to give it an address, it needs to be defined in some translation unit. And in that case, you need to separate the declaration from the definition, since otherwise it would get defined in multiple translation units.

Using g++ with no optimization (-O0), it automatically inlines constant integer variables but not constant double values. At higher optimization levels (e.g. -O1), it inlines constant doubles. Thus, the following code compiles at -O1 but NOT at -O0:

// File a.h
class X
{
 public:
  static const double d = 1.0;
};

void foo(void);

// File a.cc
#include <stdio.h>

#include "a.h"

int main(void)
{
  foo();
  printf("%g\n", X::d);

  return 0;
}

// File b.cc
#include <stdio.h>

#include "a.h"

void foo(void)
{
  printf("foo: %g\n", X::d);
}

Command line:

g++ a.cc b.cc -O0 -o a   # Linker error: ld: undefined symbols: X::d
g++ a.cc b.cc -O1 -o a   # Succeeds

For maximal portability, you should declare your constants in header files and define them once in some source file. With no optimization, this will not hurt performance, since you’re not optimizing anyways, but with optimizations enabled, this can hurt performance, since the compiler can no longer inline those constants into other source files, unless you enable “whole program optimization”.

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