Question

In the Open Babel library, there are defined many iterator objects for OBMol class, like OBMolAtomiter. On the linked page, there is following code sample illustrating the usage.

  #include <openbabel/obiter.h>
  #include <openbabel/mol.h>

  OpenBabel::OBMol mol;
  double exactMass = 0.0;
  FOR_ATOMS_OF_MOL(a, mol)
  {
     // The variable a behaves like OBAtom* when used with -> and * but
     // but needs to be explicitly converted when appearing as a parameter
     // in a function call - use &*a

     exactMass +=  a->GetExactMass();
  }

(FOR_ATOMS_OF_MOL(a, mol) expands into a for cycle, a is declared to be of the iterator type. mol is an existing molecule to iterate over)

I want to ask, why the &*p thing described in the comment is necessary. The behavior when I pass the iterator to a function that expects pointer is that the the code compiles, but the program behaves strangely.

I tried to Google for it, I found the page about iterator_traits, is it anyhow related?

Answer 1

The FOR_ATOMS_OF_MOL(a, mol) macro constructs a of type OBMolAtomIter. To return OBAtoms, the * and -> operators have been overloaded. That is why a cannot be transmitted directly to functions, but *a and a-> behave as if a was an OBAtom *.

http://openbabel.org/api/2.2.0/classOpenBabel_1_1OBMolAtomIter.shtml