Re: [boost] [iterator + lambda] transform_iterator and lambda expression

From: Daniel Wallin (dalwan01_at_[hidden])
Date: 2004-04-23 10:57:31

Vladimir Prus wrote:
> I'm again trying to use FP in C++, and having problems. What I wanted to do 
> was:
> 
>    for_each(predecesor(v, g), /* BLL expression */ );
> 
> where predecessor(..) returns a pair of 'transform_iterator' instances. The 
> simplest example which illustrates the problem is:
> 
>      struct functor {
>          typedef int result_type;
>         int operator()(int i) const { return i+1; };
>      };
> 
>      transform_iterator<functor, vector<int>::iterator> it(v.begin());
>      (cout << _1)(*it);
> 
> This does not compile, because operator* of transform_iterator return rvalue 
> which can't be bound to non-const reference that operator() of the lambda 
> expression accepts.
> 
> Of all workarounds suggsted on BLL docs, only break_const is applicable to my 
> case (I want to pass non-const references to the lambda, so const_parameters 
> won't do), and break_const is scary.
> 
> It looks like a serious problem with using FP, so I wonder what can be done. 

How about what Peter Dimov suggested here:

   http://article.gmane.org/gmane.comp.lib.boost.devel/34663/match=+cref+

Something like:

   transform_iterator<functor, vector<int>::iterator> it(v.begin());
   boost::make_adaptable<void, int>(cout << _1)(*it);

-- 
Daniel Wallin