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

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_::type, typename insert, at >::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::type >::type > { }; template< typename Map > struct toy_metafunction : eval_if< typename has_key::type , increment_key_counter , identity<Map> > { }; -- Aleksey Gurtovoy MetaCommunications Engineering