Re: [Boost-users] [typeof] Determining if a type has been registered or not at compile time

joel falcou wrote:

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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:

boost::mpl::eval_if < boost::typeof::registered<T>, // this would be nice to have boost::mpl::identity, boost::mpl::identity

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A simple way could be to add a meta-function specialisation in the typeof registration macro so that a is_registered<T> is available in a

Edward Diener wrote: pervasive way.

I don't know how the internals of typeof work, but what I think probably has to happen is that all types which are registered by the end-user need to be kept in some sort of MPL sequence which can be later checked to see whether a particular type is registered at compile time via some boolean metafunction. But I have not heard anything from the typeof developers so I do not think they are much interested in this issue. For my own use I am using typeof to generate the return values of operators for built-in types but I want to be able to do the same for user-defined types also. But if a user-defined type is not registered via typeof I want to fall back on other methods rather than the have the end-user issued a compiler error. To do this I need a metafunction which tells me whether or not a type is registered with typeof.