[Boost-users] Range library suggestion

Subject: [Boost-users] Range library suggestion
From: John M. Dlugosz (mpbecey7gu_at_[hidden])
Date: 2013-01-23 11:39:03

I just added my second metafunction of the form is_range_of_X<T> for use with enable_if, 
and although the first one took some fiddleing to pass both compilers I was using, the 
second was a clone of the first with the name and type changed. So it really ought to be a 
general purpose
is_range_of<Whatever, T> metafunction.

I would suggest that such a thing be added to the Range library.  It is meant for 
overloaded forms that operate on ranges, and you don't want to accept just any ol' thing 
because, perhaps, you want other overloads or to get better error messages for bad arguments.

A typical example would be something like:

     void insert (const C& x);

     template <typename R>
     typename enable_if<typename is_range_of<C,R>::type,
     void>::type insert (const R& range) { ⋯ }

In my case, I check whether R is indeed a Range (something that appears at first to be 
missing from the library too, but has_range_iterator is probably what is really meant by 
that) and the underlying element is convertible to the desired type.  In a general-purpose 
library you might need to put more thought into what tests are so common as to be worth 
reusing at that level.

I think having convertable-to-C (where you just give C) and another one which lets you 
pass in another metafunction for matching the element would allow reuse of experly written 
common code that takes care of nuances across compilers, e.g. choking on value_type if the 
outer is NOT a range, so you need to ensure short-circuit evaluation of the meta-expression.

—John