Re: [boost] [type_traits] Interest in is_iterator type trait?

Subject: Re: [boost] [type_traits] Interest in is_iterator type trait?
From: Peter Dimov (lists_at_[hidden])
Date: 2014-08-19 13:49:57

Howard Hinnant wrote:
> Fwiw, I used something closely related in libc++ for SFINAE on the “member 
> template members” of the containers (the ones taking iterators) to 
> disambiguate them from the members/constructors not taking iterators, but 
> having the same number of parameters.  But instead of is_iterator, I 
> needed is_input_iterator, is_forward_iterator, etc.

That was exactly what I had in mind. Instead of enable_if on is_iterator<T>, 
it's almost always better to use enable_if on is_convertible<typename 
iterator_traits<T>::iterator_category, tag>, where tag is the kind of 
iterator the function actually needs. (This requires the new and improved 
iterator_traits though - were those C++14?)

About the only use case I can envisage for the plain is_iterator is

template<class It> typename enable_if<is_iterator<T>::value>::type 
function( It first, It last )
{
    static_assert( is_forward_iterator<T>::value, "'function' requires 
forward iterators" );
    // ...
}

where you deliberately don't SFINAE out the function in order to produce a 
better error message in the presence of other overloads.

But even then... is_input_iterator would be good enough, I suppose.

(-pedantic on: an iterator is not required to have nested types, so the 
implementation is not quite correct. Of course, again, without the new and 
improved iterator_traits, it's the best that we can do.)