CGAL users discussion list

[Philipp Moeller <>]

Ross Hemsley 
<>
 writes:

> I am writing a class with a series of functions that work by applying 
> functors to an iterator range over a set of elements. 
>
>
>
>
> Example: 
>
>
> template< typename Iterator, typename Functor > 
> double mean(Iterator begin, Iterator end, Functor f) 
> { 
> double sum = 0; 
> int count = 0 ; 
>
>
> for (Iterator i=begin; i!=end; ++i) 
> { 
> sum += f(*i); 
> ++count; 
> } 
> } 
>
> However, I now want to use the "Triangulation_2::finite_vertices_iterator" 
> as the iterator, and I want to apply a functor that requires access to the 
> vertex_handle. - This is a problem because the algorithm above dereferences 
> the iterator and gives me a plain vertex: The problem is that dereferencing 
> the iterator that CGAL gives us causes us to lose information. 
>
>
> I'm looking for a solution that keeps the same input parameters (an 
> iterator 
> with '::value_type' that corresponds to the argument of the functor). 
> But may work differently 'under the hood' - i.e. we have a special 
> helper function "apply(f, x)" that applies a function to an 
> iterator. I tried to use some dirty 'template magick' tricks to do this but 
> the ideas I had require features that are only reliably available in C++11. 
>
>
> Any ideas? 

I'm think you can write the necessary trait to detect if your functor is
callable with an lvalue of a certain type in C++03. There are a some
tricky parts when it comes to const qualification and behavior for
iterators where *it doesn't return a T&, but it should be possible.

On the other hand: Your functor can always cheat a
little. Triangulation_data_structure_2 is using Compact_container (this
is even publicly documented), which allows you to get an iterator/handle
from a value through Compact_container::s_iterator_to.