Re: [Boost-users] Using single-pass ranges.

Subject: Re: [Boost-users] Using single-pass ranges.
From: Nathan Ridge (zeratul976_at_[hidden])
Date: 2012-07-25 00:13:54

>> I have a class that hands out a sequence of Things through a 
>> getNextThing() interface, ie., 
>>  
>> struct Thing { }; 
>>  
>> struct ThingFeed 
>> { 
>>      Thing * getNextThing( ); 
>> }; 
>>  
>> returning null when there's no more things. I'd like to be 
>> able to feed this into the Boost range-based algorithms, something 
>> like 
>  
> Since we are not in a position to offer range primitives, there is more  
> work required than one would like. To maximize interoperability with  
> algorithms and existing C++ code the ranges in Boost.Range are limited  
> to being implemented on top of iterators. Therefore the getNextThing()  
> would need to be part of an increment operator / operators upon a  
> forward traversal iterator. This is not difficult to implement using  
> Boost.Iterator. 

Here's how you might do such a thing (untested):

class ThingIterator : public boost::iterator_facade<ThingIterator, Thing, boost::single_pass_traversal_tag>
{
public:
   // Public interface
   ThingIterator(ThingFeed& feed, Thing* cur = NULL) : m_feed(feed), m_cur(cur) {}

private:
   // Interface to boost::iterator_facade
   void increment() { m_cur = m_feed.getNextThing(); }
   Thing& dereference() const { return *m_cur; }
   bool equal(const ThingIterator& other) const { return m_cur == other.m_cur; }

private:
   // Implementation
   ThingFeed& m_feed;
   Thing* m_cur;
};

class ThingRange : public boost::iterator_range<ThingIterator>
{
public:
    ThingRange(ThingFeed& feed)
        : boost::iterator_range<ThingIterator>(
             ThingIterator(feed, feed.getNextThing()),
             ThingIterator(feed)) 
    {}
};

...

ThingFeed feed;
...
boost::for_each( ThingRange(feed) | filter( someCondition ), doSomething );

Regards,
Nate