[boost] Re: indirecter_unary, indirecter_binary

From: Matthias Kaeppler (nospam_at_[hidden])
Date: 2005-03-07 05:07:45

Thorsten Ottosen wrote:
> Hi Matthias,
> 
> "Matthias Kaeppler" <nospam_at_[hidden]> wrote in message 
> news:d0ff4g$7rm$1_at_sea.gmane.org...
> | Hello,
> |
> | because of the lack of a generic adaptor which indirects operations on
> | pointers to the pointees, i.e. sorting a collection of pointers with
> | predicates applying to the pointees, I have come up with two classes,
> | indirecter_unary and indirecter_binary which do exactly that (thanks
> | again to the people from boost users and comp.lang.c++ for assisting
> | me). Both adaptors accept adaptable function objects as well as plain
> | functions.
> 
> these classes are useful when we need to use a class; boost.bind won't
> work since we don't know what type it returns.
> 
> I have been playing with similar functionality for Boost.Pointer Container
> because the ptr_set class needs it.
> 
> I have a question, though. I'm thinking, what is the benefit of your approach 
> compared to
> 
> template< class Fun >
> class indirect_fun
> {
>     Fun fun;
>  public:
>     typedef typename result_of<Fun>::type result;
> 
>     indirect_fun( Fun f ) : fun(f)
>     { }
> 
>     template< class T >
>     result operator( T* r ) const
>    { return fun( *r ); }
> 
>     template< class T, class U >
>     result operator( T* r, U* r2 ) const
>    { return fun( *r, *r2 ); }
> 
>     // etc
> };
> 
> br
> 
> -Thorsten 
> 
> 
> 
> _______________________________________________
> Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
> 

There are several problems with this template.

First, it's not an "Adaptable Binary Function" or "Adaptable Unary 
Function", because it fails to provide the required typedefs result_type 
and argument_type, and result_type and first_argument_type, 
second_argument_type respectively.

Second, it might fail to compile when Fun takes arguments by reference. 
I had problems at this part, that's why I used the boost type traits 
library to remove any references on the type first, and only work on 
"raw" const-pointers.

Third, (correct me if I'm wrong, I haven't used it before), as far as I 
understand, the result_of class template won't work if you instantiate 
your template with a plain function.
Instead, boost::unary_traits and boost::binary_traits take care of the 
right typedefs if you want your adaptor to work with both functors and 
functions.

-- 
Matthias Kaeppler