[C++-sig] [boost.python] Register a Python-callable with C++ code and call it from C++ code?

[Paul O. Seidon]

I want to register Python-callables with my VariableFloat-class (which is a 
subclass of a template-class Variable<double>) and call them if the 
variable's value changes.

For this I added an instance of class EventEmitter, which should hold 
references to those callbacks. So, VariableFloat delegates to EventEmitter, 
when it comes to deal with callbacks (reg'ing and calling).

template <class TYPE>
class EventEmitterSync3
{
public:

<snip/>

    void                        operator()() // Call the handler
                                {
                                    if (_p_tau4eh)
                                        (*_p_tau4eh)( *this);
                                };

    void                        register_tau4ehS( void (*callable)( 
EventEmitterSync3<TYPE>& )) // Register the handler
                                {   _p_tau4eh = callable;
                                };
<snip/>

private:
    void                        (*_p_tau4eh)( EventEmitterSync3<TYPE>& ) = 
NULL; // The handler
};


The class VariableFloat (actually its base class _Variable<TYPE>) holds a 
member of type EventEmitterSync<TYPE> like so:

private:
    EventEmitterSync3<TYPE>     _tau4ee_on_change;

and the wrapper definitions are

void    (VariableFloat::*register_tau4ehS)( void (*)( 
EventEmitterSync3<double>& )) = &VariableFloat::register_tau4ehS;

and 

        .def( "register_tau4ehS", register_tau4ehS)


Compiling yields errors coming from boost being rather cryptic to me. But 
the last few lines say:

/media/truecrypt13/D.X/Projects/DDG/tau4/src/cpp/src/tau4misc/main.cpp:43:1:   
required from here
/usr/include/boost/python/converter/registered.hpp:86:7: error: no matching 
function for call to ?register_shared_ptr1(void (*)
(EventEmitterSync3<double>&))?
/usr/include/boost/python/converter/registered.hpp:86:7: note: candidate is:
/usr/include/boost/python/converter/registered.hpp:77:3: note: 
template<class T> void 
boost::python::converter::detail::register_shared_ptr1(const volatile T*)
/usr/include/boost/python/converter/registered.hpp:77:3: note:   template 
argument deduction/substitution failed:
/usr/include/boost/python/converter/registered.hpp:86:7: note:   types 
?const volatile T? and ?void(EventEmitterSync3<double>&)? have incompatible 
cv-qualifiers

Seems I have to put "const volatile" somewhere? 

I found some dox, which could be relevant, here: 
http://www.boost.org/doc/libs/1_51_0/libs/python/doc/v2/callbacks.html

But I am not sure how to apply that info to my problem. I tried to change 
all function pointers into objects, but that didn't work.

Has anyone some sample code on storing pointers to Python functions and then 
calling them and is willing to share it?

Paul