How to extend a struct in SWIG with a static variable, with a custom getter and setter?

Question

I want to wrap a static member variable of a struct, such that when it is read or written from Python, a custom get/set function is called on the C++ side. The following works, for a non-static variable. This is foo.h :

#ifndef FOO_H
#define FOO_H

#include <iostream>

struct Foo
{
  static int bar() { std::cout << "Foo::bar()\n"; return 0; }
};

#endif

foo.i looks as follows:

%module foo
%{
  #include "foo.h"
%}

struct Foo
{
};

%extend Foo {
  int i;
}

%{
  int Foo_i_get(Foo* obj) {
    return Foo::bar();
  }
  void Foo_i_set(Foo* obj, int i) {}
%}

This sort of works, in Python I can do:

import foo
f = foo.Foo()
f.i

This triggers Foo::bar() being called. What I want to achieve is the following having the same behavior:

import foo
foo.Foo.i

This does not work, since i is not a class property. I naively tried changing the following in the .i file:

%extend Foo {
  static int i;
}

This gives compilation errors in the following wrapper code generated by SWIG:

SWIGINTERN PyObject *Swig_var_Foo_i_get(void) {
  PyObject *pyobj = 0;

  pyobj = SWIG_From_int((int)(Foo::i));
  return pyobj;
}

This is the error message given by g++ :

foo_wrap.c: In function ‘PyObject* Swig_var_Foo_i_get()’:
foo_wrap.c:3208:31: error: ‘i’ is not a member of ‘Foo’
    pyobj = SWIG_From_int((int)(Foo::i));

So obviously this is not the right approach. How can I extend a struct with a static variable, with custom setters and getters?

Answer 1

this might work: foo.i looks as follows:

%module foo
%{
  #include "foo.h"
%}

%include "foo.h"

%extend Foo {
  int get_i(){ return Foo::bar(); }
  %pythoncode %{
     i = property(get_i)
  %}
}