Question

Consider the following. I have two exported constants as follows:

// somefile.h
extern const double cMyConstDouble;
extern const double cMyConstDouble2;

and

// somefile.cpp
const double cMyConstDouble = 3.14;
const double cMyConstDouble2 = 2.5*cMyConstDouble;

These constants are now referenced some place else to define two static (locally visible) constants:

// someotherfile.cpp
#include "somefile.h"
static const double cAnotherDouble = 1.1*cMyConstDouble;
static const double cAnotherDouble2 = 1.1*cMyConstDouble2;
printf("cAnotherDouble = %g, cAnotherDouble2 = %g\n",
       cAnotherDouble, cAnotherDouble2);

Which yields the following output:

cAnotherDouble = 3.454, cAnotherDouble2 = 0

Why is the second double 0? I’m using .NET 2003 C++ compiler (13.10.3077).

Answer 1

Because cMyConstDouble is declared as extern, compiler is not able to assume its value and does not generate a compile time initialization for cMyConstDouble2. As the cMyConstDouble2 is not compile time initialized, its order of initialization relative to cAnotherDouble2 is random (undefined). See static initialization fiasco for more information.