Demonstrate referring to an argument type in the BOOST_PARAMETER_FUNCTION macro

boost_parameter_return_type.cpp

#include <boost/parameter/name.hpp>
#include <boost/parameter/preprocessor.hpp>

BOOST_PARAMETER_NAME(b)
BOOST_PARAMETER_NAME(c)

BOOST_PARAMETER_FUNCTION(
// Deduce return type from an argument type
// This is not documented, but seems like it could be officially supported:
(typename boost::parameter::value_type<Args, tag::c, int>::type),
ffff,
tag,
(required 
    (b, *))
(optional
    (c, *, 11))
)
{
    return b + c;
}

int main(int argc, char** argv) {
    std::cout << ffff(3, _c = 4) << "\n";    // return value is integer
    std::cout << ffff(3, _c = 4.5) << "\n";  // return value is floating point
    return 0;
}