Yes, I have often wondered why those were left out. Several comments:

1) Sometimes I use Boost::Tuple instead of std::pair just because it has getters and that makes for_each useable.
2) This would be an excellent point to make when discussing why getters are good programming.
3) I wrote my own templates to do this, they're very simple.

template < typename P > typename P::first_type const & const_first_of_pair(P const & p) { return p.first; }
template < typename P > typename P::first_type& first_of_pair(P& p) { return p.first; }
template < typename P > typename P::second_type const & const_second_of_pair(P const & p) { return p.second; }
template < typename P > typename P::second_type& second_of_pair(P& p) { return p.second; }

The const/non-const versions turn out to be necessary in practice. While you want the const versions for this particular application, there are other times when the std::pair is mutable and having a non-const accessor is required.

Use looks like this, for a map where I want to do an internal data update on each object in an STL container (i.e., call a non-const method on each value type in the container).

std::for_each(m_lines.begin(), m_lines.end(), bind(&Line::split, bind(&second_of_pair<container::value_type>, _1)));

Implementation issues:

1) I made them functions instead of functors because I always pass them to Boost.Bind and functors add weight without benefit in that case.
2) You have to pass in the pair type, not the STL container type, because I didn't have the energy to write the MPL logic to detect the
"container-ness" of the template argument. I think that could be done, but I didn't think about it very much.
3) Could Boost.get be extended so that you could use "get<1,container::value_type>" instead of "second_of_pair<container::value_type>"?

At 05:04 AM 7/24/2006, gast128 wrote:

Dear all,

in our company we use the STL map frequently. However one often wants to
iterate over the key_type only (or mapped type, or make desicions for the key
type, and take action on its corresponded map type).

A select1st or select2nd would already a great help.