[boost] First lambda

From: David Abrahams (dave_at_[hidden])
Date: 2004-03-03 15:31:21

I just tried using the lambda library for the first time.  Here's
what came out of it, after some struggle:

  #include <boost/lambda/lambda.hpp>
  #include <vector>
  #include <numeric>

  using namespace boost::lambda;

  struct X
  {
      X() : count(1) {}

      unsigned long count;
  };

  std::vector<X> xs(10);
  unsigned long n = std::accumulate(
      xs.begin(), xs.end(), 0UL,
      _1 + (&_2) ->* &X::count);

This program adds up the "count" members in a vector of X objects.
Now, no offense intended, but this is nasty.  There's just too much
extra syntax required for something so simple, plus you have to make
the type into a pointer just so you can dereference it Before I
realized I needed to take the address of _2, I had:

  unsigned long n = std::accumulate(
      xs.begin(), xs.end(), 0UL,
      _1 + _2 ->* &X::count);

which is only slightly better (but incorrect).  I realize that I can
use bind:

    
  unsigned long n = std::accumulate(
      xs.begin(), xs.end(), 0UL,
      _1 + bind(&X::count,_2));

but that's slightly counter-intuitive since the order of the target
object and its member pointer are reversed.  It'd be nice to allow
something like:

  unsigned long n = std::accumulate(
      xs.begin(), xs.end(), 0UL,
      _1 + _2.at(&X::count));

while if _2 were to refer to a pointer type, we could say:

  unsigned long n = std::accumulate(
      xs.begin(), xs.end(), 0UL,
      _1 + _2->at(&X::count));

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com