Question

I have a C++ value type wrapped with Boost.Python which has a concept of a NULL value. The relevant parts of the wrapper code appear as follows:

class_<TCurrency> currency( "TCurrency" )
    .def( init<long>() )
    .def( init<const std::string&>() )
    <...>;

Currently, trying to create a NULL instance in Python by passing None to the __init__() method causes the C++ ctor accepting a const string reference to be called with an invalid reference. (&arg == NULL)

Is it possible to trap the case where None is being passed to a constructor and handle it gracefully or at least to throw a meaningful exception before my program crashes?

Using Boost 1.36 and Python 2.6.2.

Answer 1

Adding an init<void*> overload will pass NULL if None is used, but I’m not sure how this could affect other ctors in corner cases. I also don’t get the same None to string const& conversion that you mention, if I leave init<void*> out. Using Boost.Python 1.37 and Python 2.6.2.

Example:

#include <iostream>
#include <string>

#include <boost/python.hpp>


struct A {
#define BODY { std::cout << __PRETTY_FUNCTION__ << '\n'; }
    A() BODY
    A(long) BODY
    A(std::string const&) BODY
    A(void* p) BODY
#undef BODY
};

BOOST_PYTHON_MODULE(ex) {
using namespace boost::python;
class_<A>("A")
    .def(init<long>())
    .def(init<std::string const&>())
    .def(init<void*>())
;
}

>>> import ex
>>> ex.A()
A::A()
<ex.A object at 0x839bf7c>
>>> ex.A(42)
A::A(long int)
<ex.A object at 0x839bfcc>
>>> ex.A("abc")
A::A(const std::string&)
<ex.A object at 0x839bf7c>
>>> ex.A(None)
A::A(void*)
<ex.A object at 0x839bfcc>

If init<void*> is left out:

>>> ex.A(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Boost.Python.ArgumentError: Python argument types in
    A.__init__(A, NoneType)
did not match C++ signature:
    __init__(_object*, std::string)
    __init__(_object*, long)
    __init__(_object*)