Named arguments in C++ (a bare bones concept)

named_args.cpp

#include <iostream>
#include <ostream>

#define DEFINE_NAMED_ARG(type, arg_name, arg_type)  \
    struct type                                     \
    {                                               \
        type(arg_type val = arg_type())             \
            : val_(val)                             \
        {}                                          \
                                                    \
        operator arg_type() const                   \
        {                                           \
            return val_;                            \
        }                                           \
                                                    \
    private:                                        \
        arg_type val_;                              \
    };                                              \
                                                    \
    struct type ## _tag                             \
    {                                               \
        type operator=(arg_type val) const          \
        {                                           \
            return type(val);                       \
        }                                           \
    };                                              \
                                                    \
    type ## _tag arg_name;                          \

DEFINE_NAMED_ARG(year_t,  year,  int);
DEFINE_NAMED_ARG(month_t, month, int);
DEFINE_NAMED_ARG(day_t,   day,   int);

void print_date(year_t year, month_t month, day_t day)
{
    std::cout << month << '/' << day << '/' << year << std::endl;
}

int main()
{
    print_date(year = 2012, month = 6, day = 7);
    print_date(2012, 12, 1);

    // won't compile:
    // print_date(month = 6, year = 2012, day = 7);
}