[C++-sig] Wrapping functions with lvalue arguments

Wagner Bruna wbruna at yahoo.com
Sun Jan 16 05:23:34 CET 2005


I'm trying to wrap some C++ functions that take some
arguments by non-const reference. I didn't want to
return a tuple in this particular case (mainly because
there'll be a lot of variables), so I tried to build a
"holder" object to store the primitive type and pass
it as an lvalue. Something like:

void my_function(int & value);

struct IntHolder {
  int obj;
  operator int&() {
    return obj;

(in theory, the implicit conversion would allow me to
wrap and use my_function directly)

But I couldn't find a way to wrap the conversion
operator to int&. I also tried a free function
returning int&, and a wrapped int member, without
sucess. What I found really strange was, although I
could wrap my_function(int&) without problems, I
couldn't find any way to actually pass an lvalue int
into it.

Well... The curious part is: after some unfruitful
tries, almost by accident I found a rather bizarre
way. Easier to show than to describe:

#include <boost/python/class.hpp>
#include <boost/python/def.hpp>
#include <boost/python/module.hpp>

int my_function(int & i) { return i++; }

// "member" functions
int get_obj(int obj) { return obj; }
void set_obj(int & obj, int v) { obj = v; }

  using namespace boost::python;

  def("my_function", &my_function);

  class_<int>("IntHolder", init<int>())
    .add_property("obj", &get_obj, &set_obj)

It is necessary to comment out the is_class static
assertion on boost/python/object/make_instance.hpp -
and I understand this implies I'm *not* doing
something recommended... anyways, it did the job:

>>> from holder import *
>>> a = IntHolder(42)
>>> a
<holder.IntHolder object at 0x01587210>
>>> a.obj
>>> my_function(a)
>>> a.obj

But is there a better (or "less hacking") way to make
this "holder" object? And are there better
alternatives to deal with functions that must return
by reference?

Thanks in advance,

pdf: FWIW, I'm using Boost.Python from release 1.32.0,
and Visual C++ 6.

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