How to expose a c++ function taking variable arguments in boost python

Question

I have a c++ function taking variable number of arguments.

   char const* Fun(int num, ...)
   {
   ....//does some processing on the arguments passed
   }

Boost Python code for exposing this function is written as,

    using namespace boost::python;
    BOOST_PYTHON_MODULE( lib_boost )
   {
       def( "Fun", Fun );
   }

while compiling this code gives the below error

In file included from /boost_1_42_0/boost/python/data_members.hpp:15, from /boost_1_42_0/boost/python/class.hpp:17, from /boost_1_42_0/boost/python.hpp:18, from Lib_boost.h:3, from Lib_boost.cpp:1: /boost_1_42_0/boost/python/make_function.hpp: In function 'boost::python::api::object boost::python::make_function(F) [with F = const char* ()(int, ...)]': /boost_1_42_0/boost/python/def.hpp:82:
instantiated from 'boost::python::api::object boost::python::detail::make_function1(T, ...) [with T = const char ()(int, ...)]' /boost_1_42_0/boost/python/def.hpp:91: instantiated from 'void boost::python::def(const char, Fn) [with Fn = const char* ()(int, ...)]' Lib_boost.cpp:540: instantiated from here /boost_1_42_0/boost/python/make_function.hpp:104: error: invalid conversion from 'const char ()(int, ...)' to 'const char ()(int) /boost_1_42_0/boost/python/make_function.hpp:104: error:
initializing argument 1 of 'boost::mpl::vector2 boost::python::detail::get_signature(RT ()(T0), void*) [with RT = const char*, T0 = int]'

My understanding from the error info above is boost python could not recognize the function taking variable arguments(invalid conversion from 'const char* ()(int, ...)' to 'const char (*)(int)')

Exposing a function with fixed/known set of arguments is not the same for functions taking variable arguments. How to expose a function with variable arguments?

Answer 1

if you are using c++11 then following could work ( tested on g++-4.8.2 )

#include <boost/python.hpp>
#include <boost/python/list.hpp>
#include <vector>
#include <string>
#include <cstdarg>
#include <cassert>

using namespace boost::python;

template <int... Indices>
struct indices
{
    using next = indices<Indices..., sizeof...(Indices)>;
};

template <int N>
struct build_indices
{
    using type = typename build_indices<N-1>::type::next;
};

template <>
struct build_indices<0>
{
    using type = indices<>;
};
template <int N>
using BuildIndices = typename build_indices<N>::type;

template <int num_args>
class unpack_caller
{
private:
    template <typename FuncType, int... I>
    char * call(FuncType &f, std::vector<char*> &args, indices<I...>)
    {
        return f(args.size(), args[I]...);
    }

public:
    template <typename FuncType>
    char * operator () (FuncType &f, std::vector<char*> &args)
    {
        assert( args.size() <= num_args );
        return call(f, args, BuildIndices<num_args>{});
    }
};

//This is your function that you wish to call from python
char * my_func( int a, ... )
{
    //do something ( this is just a sample )
    static std::string ret;

    va_list ap;
    va_start (ap, a);
    for( int i = 0; i < a; ++i)
    {
        ret += std::string( va_arg (ap, char * ) );
    }

    va_end (ap);
    return (char *)ret.c_str();
}

std::string my_func_overload( list & l )
{
    extract<int> str_count( l[0] );
    if( str_count.check() )
    {
        int count = str_count();
        std::vector< char * > vec;
        for( int index = 1; index <= count; ++index )
        {
            extract< char * > str( l[index] );
            if( str.check() )
            {
                //extract items from list and build vector
                vec.push_back( str() );
            }
        }
        //maximum 20 arguments will be processed.
        unpack_caller<20> caller;

        return std::string( caller( my_func, vec ) );
    }

    return std::string("");
}

BOOST_PYTHON_MODULE(my_module)
{
    def("my_func", my_func_overload )
    ;
}

In python:

Python 2.7.6 (default, Mar 22 2014, 22:59:38) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import my_module as m
>>> m.my_func([5, "my", " first", " five", " string", " arguments"])
'my first five string arguments'
>>>

In this example "char * my_func( int a, ... )" simply concatenates all the string arguments and returns the resulting string.

Answer 2

I find the best way to treat variadic arguments is by using raw_function. This way you get full control in converting your C++ parameters into Python objects:

The wrapper:

using namespace boost::python;

object fun(tuple args, dict kwargs)
{
    char* returned_value;

    for(int i = 0; i < len(args); ++i) {
        // Extract the args[i] into a C++ variable,
        // build up your argument list
    }

    // build your parameter list from args and kwargs 
    // and pass it to your variadic c++ function

    return str(returned_value);
}

The declaration:

def("fun", raw_function(fun, 1) );

raw_function takes two arguments: the function pointer and minimum number of arguments.

Answer 3

You can probably do it by treating the arguments as optional, so long as you know what the maximum count can be. See here: https://wiki.python.org/moin/boost.python/FunctionOverloading