Re: [Boost-users] [boost-users] istream_iterator question

Subject: Re: [Boost-users] [boost-users] istream_iterator question
From: Dmitry Bufistov (dmitry_at_[hidden])
Date: 2009-05-10 15:10:17

Max escribió:
> Hello,
> 
> I don't know why the following code does not work as expected.
> I hope the guru's here could lend me a hand.
> 
> For a test data file:
> 3
> 1 2
> 3 4
> 5 6
> 
> 2 
> 1.2 4.3 234.2
> 3.5656 67.88 345.3
> <eof>
> 
> and 2 simple test helper class:
> 
> struct a
> { int i, j; };
> std::istream& operator>>(std::istream& in, a & p)
> {  in >> p.i >> p.j;  return in;  }
> std::ostream& operator<<(std::ostream& out, const a & p)
> {  out << p.i << " " << p.j; return out; }
> 
> struct b
> { float i, j, k; };
> std::istream& operator>>(std::istream& in, b & p)
> {  in >> p.i >> p.j >> p.k; return in;  }
> std::ostream& operator<<(std::ostream& out, const b & p)
> {  out << p.i << " " << p.j << " " << p.k; return out;  }
[Snip]
> 
> copy_n is defined as:
> 
> template <class InputIterator, class Size, class OutputIterator>
> 	OutputIterator copy_n( InputIterator first, InputIterator last,
> 	Size n,
> 	OutputIterator result) 
> {
> 	// copies the first `n' items from `first' to `result'. Returns
> 	// the value of `result' after inserting the `n' items.
> 	// it ends before n iterations, if the last iterator is reached
> 	while( n-- && first != last)
> 	{
> 		*result = *first;
> 		++first;
> 		++result;
> 	}
> 	return result;
> }

Change your copy_n() as follows:

template <class InputIterator, class Size, class OutputIterator>
        OutputIterator copy_n( InputIterator first, InputIterator last,
        Size n,
        OutputIterator result)
{
        // copies the first `n' items from `first' to `result'. Returns
        // the value of `result' after inserting the `n' items.
        // it ends before n iterations, if the last iterator is reached
        while( n-- && first != last)
        {
                *result = *first;
                if (n) ++first;
                ++result;
        }
        return result;
}

Regards,
Dmitry