Re: [Boost-users] Re: boost::function - why does this work??

From: Peter Dimov (pdimov_at_[hidden])
Date: 2003-11-28 07:02:01

Matt S Trentini wrote:
> Peter Dimov wrote:
>
>> When you construct a boost::function from &HandleClass::handle, what
>> actually gets stored is boost::mem_fn(&HandleClass::handle). See
>>
>>     http://www.boost.org/libs/mem_fn/
>>
>> Even
>>
>>      std::vector< boost::shared_ptr<HandleClass> > listOfHcs;
>>
>> will work.
>
> Thanks Peter, makes sense after reading that documentation now.
>
> I've actually got another question - I'd really prefer to replace
> ReturnContainer with an output iterator type.  So Extract would look
> like this:
>
>      template
>      <
>          class InIterator,
> class OutIterator,
>      >
>      void Extract(const InIterator & itBegin,
>                   const InIterator & itEnd,
>                   OutIterator      & itOut,
>                   boost::function1<OutIterator::value_type,
> FwdIterator::value_type> f)
>      {
>          for (FwdIterator it = itBegin ;it != itEnd; ++it)
>          {
>              *itOut++ = f(*it);
>          }
>      };
>
> And then you'd typically call Extract with an inserter:
>
>      Extract(listOfHcs.begin(), listOfHcs.end(),
> back_inserter(needToFill), &HandleClass::handle);
>
> But this doesn't compile because the inserters (at least on my
> platform) don't define value_type.

They aren't required to. You should use
std::iterator_traits<It>::value_type.

[...]

> Am I overlooking something here?

1. Iterators are typically passed by value.
2. You don't need to use boost::function in a template. It is typically only
used when you want a fixed compile-time signature.

template<class InIt, class OutIt, class F>
void Extract(InIt first, InIt last, OutIt out, F f)
{
    for (; first != last; ++first, ++out)
    {
        *out = f(*first);
    }
}

Now look at std::transform. :-)

(You'll no longer be able to pass &X::f directly, though, use mem_fn
explicitly.)