[Boost-users] re: copy_if

From: Witz (witz_at_[hidden])
Date: 2003-09-15 12:51:24

 > > template<class II, class OI, class PRED>
 > > std::pair<II,OI> copy_if(II ibegin, II iend, OI obegin, OI oend,
 > >                          PRED p)
 > > {
 > > 	for(; ibegin != iend; ++ibegin)
 > > 	{
 > > 		if(p(*ibegin))
 > > 		{
 > > 			*obegin = *ibegin;
 > > 			if(++obegin == oend)
 > > 				break;
 > > 		}
 > > 	}
 > > 	return std::make_pair(ibegin, obegin);		
 > > }
 > >
 >
 > This should check whether obegin == end before copying, in case obegin
 > == oend initially.  I don't know whether it should stop as soon as it
 > finds obegin == oend or wait until it knows there's another item to be
 > copied.  The former behaviour saves a little time in the case that no
 > more items are wanted once the output buffer is full.  The latter
 > behaviour avoids the occasional need for another pass that copies
 > nothing, if copy_if is called repeatedly to fill an output buffer which
 > is then flushed.

 > Ben.

I've tended to rely on the common sense of the user (after all i cannot 
detect other possible errors such as 'oend < obegin') but i suppose the 
following addition at the start would seem sensible:

ASSERT(obegin != oend);

As for your second point - the amount of time saved by 'the former 
behaviour' depends on how expensive the predicate is and how many times it 
will be called if no more satisfactory items are available. As the entries 
in the output buffer are overwritten as and when required there is no need 
in any general sense to clear it between calls and this surely obviates any 
advantage your proposed approach may have.

Witz