2.16.3 Global terms, atoms, and functors

Sometimes it is convenient to put constant terms and atoms as global variables in a file (with a static qualifier), so that they are only created (and looked up) cone. This is fine for atoms and functors, which can be created by something like this:

static PlAtom ATOM_foo("foo");
static PlFunctor FUNCTOR_ff_2("ff", 2);

C++ makes no guarantees about the order of creating global variables across “translation units” (that is, individual C++ files), but the Prolog runtime ensures that the necessary initialization has been done to allow PlAtom and PlFunctor objects to be created. However, to be safe, it is best to put such global variables inside functions - C++ will initialize them on their firstuse.

Global Terms need a bit of care. For one thing, terms are ephemeral, so it is wrong to have a PlTerm static variable - instead, a PlRecord must be used, which will provide a fresh copy of the term using PlRecord::term(). There is no guarantee that the Prolog runtime has initialized everything needed for creating entries in the recorded database (see Recorded database). Therefore, global recorded terms must be wrapped inside a function. C++ will call the constructor upon first use. For example:

static PlTerm
term_foo_bar()
{ static PlRecord r(PlCompound("foo", PlTermv(PlTerm_atom("bar"))).record());
  return r.term();
}