Question

How can you call a Function over some part of a container, using for_each() ?

I have created a for_each_if() to do a

for( i in shapes )
    if( i.color == 1 )
        displayShape(i);

and the call looks like

for_each_if( shapes.begin(), shapes.end(),
                       bind2nd( ptr_fun(colorEquals), 0 ),
                       ptr_fun( displayShape ) );

bool colorEquals( Shape& s, int color ) {
    return s.color == color;
}

However, I feel immitating STL-like algorithms is not something that I should be doing.

1. Is there a way to use only existing STL keywords to produce this ?

   I did not want to do a

    for_each( shapes.begin(), shapes.end(),
                      bind2nd( ptr_fun(display_shape_if_color_equals), 0 ) );
   

   because, in a more complicated case, the functor name would be misleading with respect to what the functor

2. *Is there a way to access a struct’s member (like colorEquals) for functions like for_each without having to create a function ? *

Answer 1

To use a regular for_each with an if you would need a Functor that emulates an if condition.

#include <algorithm>
#include <vector>
#include <functional>
#include <iostream>
#include <boost/bind.hpp>

using namespace std;

struct incr {
  typedef void result_type;
  void operator()(int& i) { ++i; }
};

struct is_odd {
  typedef bool return_type;
  bool operator() (const int& value) {return (value%2)==1; }
};


template<class Fun, class Cond>
struct if_fun {
  typedef void result_type;
  void operator()(Fun fun, Cond cond, int& i) {
    if(cond(i)) fun(i);
  }
};


int main() {
  vector<int> vec;
  for(int i = 0; i < 10; ++i) vec.push_back(i);

  for_each(vec.begin(), vec.end(), boost::bind(if_fun<incr, is_odd>(), incr(), is_odd(), _1));
  for(vector<int>::const_iterator it = vec.begin(); it != vec.end(); ++it)
    cout << *it << " ";
}

Unfortunately my template hackery isn’t good enough to manage this with bind1st and bind2nd as it somehow gets confusing with the binder being returned being a unary_function but it looks pretty good with boost::bind anyhow. My example is no means perfect as it doesn’t allow the Func passed into if_fun to return and I guess somebody could point out more flaws. Suggestions are welcome.