Re: [Boost-users] [range] filtered dereferences underlying range elements twice

Subject: Re: [Boost-users] [range] filtered dereferences underlying range elements twice
From: Akira Takahashi (faithandbrave_at_[hidden])
Date: 2011-05-31 02:07:19

Hi, Nathan.

Oven Library has solution for this problem.
oven::memoized adaptor:

http://p-stade.sourceforge.net/oven/doc/html/oven/range_adaptors.html#oven.range_adaptors.memoized

#include <iostream>
#include <vector>
#include <boost/range/adaptor/transformed.hpp>
#include <boost/range/adaptor/filtered.hpp>
#include <boost/range/algorithm/copy.hpp>
#include <pstade/oven/memoized.hpp>
int transform(int i)
{
   std::cout << "calling transform() on " << i << "\n";
   return i + 1;
}
bool filter(int i)
{
   return i % 2 == 0;
}
int main()
{
   int a[] = {1, 2, 3, 4, 5, 6, 7, 8};
   int b[8];

   boost::copy(a | boost::adaptors::transformed(transform)
                 | pstade::oven::memoized
                 | boost::adaptors::filtered(filter),
               b);

   return 0;
}

results:
calling transform() on 1
calling transform() on 2
calling transform() on 3
calling transform() on 4
calling transform() on 5
calling transform() on 6
calling transform() on 7
calling transform() on 8

2011/5/31 Nathan Ridge <zeratul976_at_[hidden]>:
>

> As you can see, the transform function is called twice for elements
> that pass the filter.
>
> Can this be avoided? In my real use case, the transformation
> performs a database lookup, and it should not be done twice.
>
> Thanks,
> Nate.

>>========================
Akira Takahashi
mailto:faithandbrave_at_[hidden]
blog:http://d.hatena.ne.jp/faith_and_brave/