[boost] Re: lambda: missing result deduction

From: Andy Little (andy_at_[hidden])
Date: 2004-09-02 07:36:09

"David Abrahams" <dave_at_[hidden]> wrote

> I had to add this to get some tests for the TMP book to pass:
>
>     namespace boost { namespace lambda {
>       template<int N>
>       struct
>       plain_return_type_2<arithmetic_action<plus_action>, char[N],
std::string> {
>          typedef std::string type;
>       };
>     }} // namespace boost::lambda
>
> Obviously this could be generalized for basic_string and for the
> reversed arguments.

The binary_operation version would  look something  like this with the its
new enable_if functionality
Zounds!  this enable_if is Fun :-) ... perhaps could be done in Lambda?,

    template <typename L, typename R>
    struct binary_operation<
        L,std::plus,R,
        typename boost::enable_if<
            boost::mpl::and_<
                boost::mpl::or_<
                    is_string<L>,
                    is_string<R>
                >,
                is_string_concatable<L>,
                is_string_concatable<R>,
                boost::is_same<
                    typename pqs::meta::to_value_type<L>::type,
                    typename pqs::meta::to_value_type<R>::type
                >
            >
        >::type
    >{
        typedef  typename boost::mpl::if_<
            is_string<L>,
            L,
            R
        >::type result_type;
   };

regards
Andy Little