Fw: [boost] iterator_adaptors

From: Arun Sivaramakrishnan (aruns_at_[hidden])
Date: 2001-01-21 00:16:51

The mail I send to John last night..
-arun
>
> ----- Original Message -----
> From: "John E. Potter" <jpotter_at_[hidden]>
> To: <boost_at_[hidden]>
> Sent: Monday, January 01, 2001 7:14 AM
> Subject: Re: [boost] iterator_adaptors
>
> > This is a lack of documentation.  Projection_iterator has reference
> > semantics always.  There is a general problem with reference template
> > parameters.  Vector<int&>?  Set<int&>?  They just don't work.  But,
> >
> > struct RefFunc : public unary_function<SomeThing, int> {
> >    int& operator() (SomeThing& st) { return st.someIntField; }
> >    };
> >
> > works fine with both std::unary_function and projection_iterator.  It
> > is a lie which works well.  The only purpose of std::unary_function is
> > to extract the correct value_type.  Reference and pointer are always
> > value_type& and value_type*.  The function object must have reference
> > semantics.
> >
>
>
>
>    Most often ,I wouldnt want to write a functor everytime I use
> transform_adaptor(or projection_adaptor)
>  Now, when I try using transform/projection_adaptor in conjunction with
the
> binder1st,binder2nd etc ,type information is extracted in a manner
> that causes compiler errors.For example,the following wont compile.
>
>  typedef boost::array<double,3>  position3;
>  typedef std::vector<position3>::iterator position_iter;
>
>  void foo(position_iter first)
> {
>     typedef  boost::mem_fun1_ref_t<position3::reference
>               ,position3,position3::size_type > mem_fun1_ref_t;
>
>    typedef  boost::binder2nd<  mem_fun1_ref_t> Function;
> //////Function is a unary_function now with first_argument_ty =double&.
>
>   typedef  position3::reference/*this is double&*/
>  (position3::*PFTYPE)(position3::size_type);
>
>    typedef boost::projection_iterator<Function,    position_iter,
>                  boost::iterator<std::random_access_iterator_tag, vtype>
>                 >::type coord_iterator;
>
>   coord_iterator first(ibegin,
>     Function(boost::mem_fun_ref((PFTYPE)&position3::operator[]),0)
>    );
>   //// etc
> }
>
> Regards,
> arun
>