[Swig-user] additional constructors in %extend and implicitconv

[Dieter V. <dva...@ax...>]

Hi everyone, i've run into something that caught me by surprise in terms
of implicit conversions and additional constructors provided through %
extend.  I experienced this issue in wrapping a C++ library to python.

The basic issue is this:
I have three classes A, B, C.
A has only the default constructor A()
B has a constructor B(const A &input)
C has a constructor C(const B &input)
and using %extend i add 
C(const A &input)

i also have %implicitconv B;

here's some python code:
aobj = A()
cobj = C(aobj)

the last line first converts aobj to an instance of B,
then uses the C(const B&) constructor.  

This is surprising since if C(const A&input) had been part of the
original C class, then that's the constructor that would have been
chosen.
Is this the expected behavior?  The reason this has come up is i have a
bunch of custom container classes in my API, some of which can convert
from one to another.  I'm now trying to add additional constructors
through %extend that take python containers (dict, set, list, tuple, or 
any iterable) so each class has an additional 'constructor' each
accepting just a PyObject * as an argument.
Obviously i could convert these to static factory methods in each class
but the constructor would have been nicer syntax.

thanks for any help/suggestions.

Here's a header file containing the C++ classes:

#include <iostream>

class A {
};

class B {
   public:
      B(const A &) {
         std::cout << "Constructing B from A" << std::endl;
      }
};

class C {
   public:
      C() { std::cout << "Empty constructor" << std:: endl;}
      C(const B &) {
         std::cout << "Constructing C from B" << std::endl;
      }
};



Here's the .i file:


%module testsample2

%{
#include <iostream>
#include "sample2.h"
%}


class A {
};

%implicitconv B;

class B {
   public:
      B(const A &);
};

class C {
   public:
      C(const B &);
};

%extend C {
   C(const A&) {
    std::cout << "Constructing C from A" << std::endl;
    return new C();
   }
};


And here's what i did to compile things:
swig -c++ -python testsample2.i
g++ -I/usr/include/python2.6 -c testsample2_wrap.cxx -o testsample2_wrap.o
g++ -shared testsample2_wrap.o -o _testsample2.so

In python execute the following:
import testsample2
a = testsample2.A()
c = testsample2.C(a)

and i see this output:
Constructing B from A
Constructing B from A
Constructing C from B