[C++-sig] Simple , , I think .. type conversion question C++ -> python

[Tim Couper]

OK. Here's what I've been stuck with all today .. I have a 3rd party C++ 
program function which returns a boost::variant  (and its inverse)

my_variant my_variant_of_string(const std::string& str)

This one takes a string & returns a variant, and am trying to wrap this 
in python, so that there I can have

 >>> my_variant_of_string('hello')
'hello'

 >>> my_variant_of_string('1.2')
1.2

 >>> my_variant_of_string('10')
10

Simple, eh? .. The reduced code below compiles, creating the python 
function correctly, but I have not successfully defined 
to_python_value/to_python_converter statement(s), so the python 
predictably fails with a ".. no to_python .. converter for .. 
boost::variant .." when run.

I can find lots of helpful references to such converters of classes (eg 
http://misspent.wordpress.com/2009/09/27/how-to-write-boost-python-converters/),
but cannot see how I should use the to_python_value functionality to 
express the "convert the output of this function to a string, int or 
double as appropriate".

Thanks

Tim "novice, but learning" Couper


-- my_variant.cpp --

#include <sstream>

typedef boost::variant<int, double, std::string> my_variant;

template <class T>

     bool from_string(T& t,const std::string& s, std::ios_base& 
(*f)(std::ios_base&))
     {
         std::istringstream iss(s);
         return !(iss >> f >> t).fail();
      }

namespace var {

     my_variant my_variant_of_string(const std::string& str)
        // return a float, int or string depending on input parameter 
content
       {
         my_variant param;

         double dval; int ival;
         if (str.find(".") != std::string::npos) {
             if (from_string<double>(dval,str,std::dec)) {
                 param = dval;
             }
         }
         else if (from_string<int>(ival,str,std::dec) ) {
             param = ival;
         }
         else {
             param= str;
         }
         return param;
      }

}// var

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

BOOST_PYTHON_MODULE(var)
{
     using namespace boost::python;

     def("my_variant_of_string", &var::my_variant_of_string);

}

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