extern variable always 0

Well, a surprising result, never use your extern variables inside a function that runs before main.

Here’s an example:

In the header:

//extern test.  the externs are giving 0's.
struct Vector4f
{
  float x,y,z,w;
  Vector4f():x(0),y(0),z(0),w(0){}
  Vector4f( float ix, float iy, float iz, float iw ) :x(ix),y(iy),z(iz),w(iw){}
} ;

extern Vector4f Red ; // This is the weird variable..

In the main.cpp file:

static map<char, Vector4f> initMapping()
{
  map<char, Vector4f> mp ;
  mp.insert( make_pair( 'r', Red ) ) ;
  
  for( auto &p : mp )
    printf( "%c: %f %f %f %f\n", p.first, p.second.x, p.second.y, p.second.z, p.second.w );
  return mp ;
}

//Vector4f Red(1,0,0,1) ; // if this is here, Red has (1,0,0,1)
map<char, Vector4f> colorMapping = initMapping() ;
Vector4f Red(1,0,0,1) ; // if Red is only given its value HERE, when the static ctor runs, Red will be (0,0,0,0)

int main(int argc, const char * argv[])
{
  for( auto &p : colorMapping )
    printf( "%c: %f %f %f %f\n", p.first, p.second.x, p.second.y, p.second.z, p.second.w );
}

So those 2 declarations of Vector4f Red (without extern qualifier) in the cpp are the issue here. If the compiler runs into the Vector4f Red(1,0,0,1) ; BEFORE running SomeStruct::initMapping(), then the mapping has the correct value for Red in it. If the compiler first runs into Vector4f Red(1,0,0,1) only __after__ running SomeStruct::initMapping(), then the value of Red inside the map would be all 0’s.