Re: [Boost-users] remove_if and transform_iterator

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-12-13 10:45:22

Josue Gomes wrote:
> Hi,
>
> Can transform_iterator be used in conjunction with remove_if
> algorithm? The compiler (vc7.1, vc8, g++) is refusing to compile this:
>
> #include <string>
> #include <utility>
> #include <vector>
> #include <functional>
> #include <cassert>
> #include <iostream>
> #include <boost/iterator/transform_iterator.hpp>
>
> std::vector<std::pair<std::string, int> > c;
>
> struct to_num {
> typedef int result_type;
>
> int operator()(const std::pair<std::string, int>& p) const
> {
> return p.second;
> }
> };
>
> int main()
> {
> using std::make_pair;
>
> c.push_back(make_pair("A", 2));
> c.push_back(make_pair("A", 3));
> c.push_back(make_pair("A", 4));
>
> c.push_back(make_pair("A", 15));
> c.push_back(make_pair("A", 16));
> c.push_back(make_pair("A", 17));
> assert(c.size() == 6);
>
> std::remove_if(
> boost::make_transform_iterator(c.begin(), to_num()),
> boost::make_transform_iterator(c.end(), to_num()),
> std::bind1st(std::less_equal<int>(), 10));

Are you sure that you want this, even if it did compile? If you apply 
remove_if+transform_iterator on:

A, 2
B, 3
C, 4
D, 15
E, 16
F, 17

you'll get

A, 15
B, 16
C, 17
D, --
E, --
F, --

where -- denotes an unspeficied value.

If this is the desired result, try

struct to_num
{
    typedef int & result_type;

    int & operator()(std::pair<std::string, int>& p) const
    {
        return p.second;
    }
};

If, on the other hand, you want

D, 15
E, 16
F, 17
--, --
--, --
--, --

you don't need transform_iterator at all:

std::remove_if( c.begin(), c.end(),

    boost::bind( std::less_equal<int>(), boost::bind( to_num(), _1 ), 10 )

);

(with the original to_num). You can also use &std::pair<std::string, 
int>::second instead of to_num() and <= 10 instead of less_equal:

    boost::bind( to_num(), _1 ) <= 10

or

    boost::bind( &std::pair<std::string, int>::second, _1 ) <= 10