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

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2005-10-04 23:22:41

Geoffrey Romer writes:
> I'm struggling with a problem of how to correctly use conditionals in
> MPL. In a nutshell, the problem is that the compiler greedily
> evaluates all parts of a conditional, even the branch that isn't
> followed. This is a problem not just for performance, but because
> sometimes the unused branch might not compile.
>
> 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.

Correct.

>
> How do I resolve this problem?

The most straightforward solution is to factor out the computation
into a separate auxiliary metafunction...

>
> 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;
> };

... like this:

    template< typename Map, typename Key >
    struct increment_key_counter
        : insert< 
              Map
            , pair<
                  Key
                , typename plus< int_<1>, typename at<Map,Key>::type >::type
                >
    {
    };

    template< typename Map >
    struct toy_metafunction
        : eval_if<
              typename has_key<Map, char>::type
            , increment_key_counter<Map, char>
            , identity<Map>
            >
    {
    };

-- 
Aleksey Gurtovoy
MetaCommunications Engineering