Re: [boost] Re: functors for taking apart std::pair?

[Daniel Wallin]

At 22:51 2003-07-10, Marshall Clow wrote:
At 7:21 AM -0400 7/10/03, David Abrahams wrote:
Marshall Clow <[EMAIL PROTECTED]> writes:

 > So, here they are. Are they useful to anyone else? Is there some 
reason that
 they don't already exist? Did I miss them somewhere?

 template <class T1, class T2>
 struct first: std::unary_function< std::pair <T1, T2>, T1>
 {
        T1 operator()(const std::pair <T1, T2> & x) const { return 
x.first;}
 };

 template <class T1, class T2>
 struct first: std::unary_function< std::pair <T1, T2>, T2>
 {
        T2 operator()(const std::pair <T1, T2> & x) const { return 
x.second;}
 };

But other than that, what do people think?
I would prefer "select_nth".

I needed them because someone had a map<int, Foo*>, and I wanted to call a 
member function
(call it "Bar") on each Foo* in the map. Turns out, that with 
boost::compose and 'second', it
was simple:

        std::for_each ( m.begin (), m.end (),
                boost::compose_f_gx ( std::mem_fun ( Foo::Bar ), second 
<int, Foo *> ()));
How about:

std::for_each(m.begin(), m.end(),
  boost::compose_f_gx(std::mem_fun(Foo::Bar), select_nth<1, std::pair<int, 
Foo*> >()));

See
http://www.cs.umu.se/~ens01dwn/select_nth.hpp
for a sample implementation that handles boost::tuples and pairs.
---
Daniel Wallin
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