Re: [boost] library for type erasure

From: Steven Watanabe (steven_at_[hidden])
Date: 2006-08-03 15:13:45

AMDG

Alexander,

>For the latter I'm thinking of the following approach:
>
>    struct plus : function<plus, anyT (anyT const&, anyT const&)>
>    {
>        int    entry(id<1>, int,    int) const;
>        double entry(id<2>, double, int) const;
>        // ...
>        std::string entry(id<100>, std::string const&, char) const;
>        // ...
>    };
>
>  
>
How do you detect the end of the entries?
I can't think of anything better in general but here are
some other possibilities that might be more convenient:

    struct plus : function<plus, anyT (anyT const&, anyT const&)>
    {
        typedef boost::mpl::vector<
            int (int, int),
            double (double, int),
            //...
            string (string const&, char),
            //...
        > entry_functions;

        template<class T, class U>
        T entry(T t, U u) const;
    };

or this:

    struct plus : function<plus, anyT (anyT const&, anyT const&)>
    {
        typedef boost::mpl::vector<int, double, string const&> 
first_any_argument_types;
        typedef boost::mpl::vector<int, char> second_any_argument_types;

        template<class T, class U>
        T entry(T t, U u) const;
    };

Of course this one only works when you want every
possible combination of the types.

In any case I agree that there should be a backup to use if
the types passed in don't match one of the signatures provided.
Maybe something like allowing any one of the arguments
to be unbounded.

    typedef boost::mpl::vector_c<int, 1, 2> 
possibly_unbounded_any_arg_indexes;

Another way would be to require all the types to be the same
when at least one of them doesn't match.

In Christ,
Steven Watanabe