Re: [Boost-users] [mpl] trouble with conditionals

From: Cromwell Enage (sponage_at_[hidden])
Date: 2005-10-04 15:25:07

--- Geoffrey Romer wrote:
> I know that eval_if is supposed to address this
> problem, but unless I'm missing something, it
doesn't
> seem to be a complete solution.  It delays
evaluating
> the branches until one of them is selected, but the
> compiler still evaluates the *arguments* to both
> branches.  If the arguments themselves don't
compile,
> I'm screwed, despite the fact that the parts of my
> code that are actually relevant (the branches that
> get used) are perfectly correct.
> 
> How do I resolve this problem?

The arguments will be evaluated if they have typename
metaf_<...>::type syntax inside them.

> Here's a toy example, a metafunction which, given a
> map, finds the value associated with the key "char"
> and, if it exists, increments the associated value
> (which we assume is numeric):
> 
> template <typename m_map>
> class toy_metafunction
> {
>     typedef typename
>     if_<typename has_key<m_map, char>::type,
> 	typename insert<m_map, pair<char,
> 				    typename plus<int_<1>,
> 						  at<m_map, char>
>                                                 
> >::type
>                                    >
>                        >::type,
> 	m_map
> 	>::type type;
> };

Try:

template <typename m_map>
struct toy_metafunction
{
    typedef typename eval_if<
                has_key<m_map,char>
              , insert<
                    m_map
                  , pair<
                        char
                      , plus<
                            int_<1>
                          , at<m_map,char>
                        >
                    >
                >
              , m_map
            >::type
            type;
};

                              HTH,
                              Cromwell D. Enage

                
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