Re: boost::lambda::select2nd

From: Markus Werle (yg-boost-users_at_[hidden])
Date: 2003-06-25 03:08:11

Jaakko Jarvi wrote:

>   std::for_each(M.begin(), M.end(), 
>   std::cout << l::bind(select2nd(), l::_1) << " ");

Well, that's too complicated ;-)
_binding_ to a function or functor is the approach 
I wanted to avoid.

I think that select1st and select2nd (or - to be generic - select<n>)
could be directly supported by boost::lambda.
It's so common to have a list/vector of tuples or a std::map 
(yes, its a request for a feature ;-))

I tried to write something taking ll_static_cast as an example,
but failed to quickly understand the requirements
and semantics of lambda_functor*

Could You correct/complete the code below?
I could not figure out how select2nd_action::apply
gets its RET defined :-( 
Is there some documentation on the functor conventions
or on how to extend LL?

class select2nd_action 
{
public:
  template<class RET, class Arg1>
  static RET apply(Arg1 &a1) {
    return a1.second;
  }
};

template <class Arg1>
inline const lambda_functor<
  lambda_functor_base<
    action<1, select2nd_action >, 
    tuple<typename const_copy_argument <const Arg1>::type>
  > 
>
ll_select2nd(const Arg1& a1) { 
  return 
    lambda_functor_base<
      action<1, select2nd_action >, 
      tuple<typename const_copy_argument <const Arg1>::type> 
    >
  ( tuple<typename const_copy_argument <const Arg1>::type>(a1));
}

best regards, 

Markus

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