Indeed :

FOO.hxx:

TYPE FOO_TEMPLATE () {

  TYPE foo;
  /*...*/
  return foo;

}

Foo.h:

#DEFINE FOO_TEMPLATE foo_int

#DEFINE TYPE int

#include "Foo.hxx"

#DEFINE FOO_TEMPLATE foo_float

#DEFINE TYPE float

#include "Foo.hxx"

It's a kind of weird and ugly macro trick, bug it can be useful in C when one's need polymorphism and performance (and cannot use C++ at all, which is a far better solution).

[Edit:] I realize now it's not polymorphism like said in the article, because the caller had to tell explicitly which foo_xxx to call. But it did provide "polymorphic" code in Foo.hxx (so only one implementation, no repetition). That said, a cast is also explicit.