Peder Holt wrote:
2009/12/22 Edward Diener <eldiener@tropicsoft.com>
Is there any way one can determine whether a type has been registered or not with Boost.Typeof at compile time so that the information can be used in TMP ? I am thinking, of course, of the use of typeof in a template, where a template parameter may or may not be aregistered type when the template has been instantiated. I would like to provide my own TMP fallback code if the type has not been registered rather than see an immediate compiler error generated. Something like:
Here is a possible implementation of boost::typeof::is_registered: snipped...
boost::mpl::eval_if < boost::typeof::registered<T>, // this would be nice to have boost::mpl::identity<BOOST_TYPEOF_TPL(some_expression)>, boost::mpl::identity</* use another facility to get the type I want into a metafunction*/>
The problem is that BOOST_TYPEOF_TPL is evaluated even if is_registered is false.
You are right and I had not realized this until you mentioned it. So 'registered' metafunction does little good for my proposed situation.
One way to workaroun this is to allow the user to add a default type: typedef BOOST_TYPEOF_WITH_DEFAULT(some_expression,some_backup_type_if_some_expression_not_registered) type;
Is this acceptable?
Yes, that seems the best solution. An excellent idea. Should I create a Trac suggestion for it or will you remember to implement it if you decide to do it ? If you do this, I think you will want to do it for TYPEOF_TPL also. I see much less use for AUTO and AUTO_TPL defaults if the type is not registered but I think you should also implement it for these two also both to be orthogonal and for the possibility that someone will find a use there also.