Re: [boost] [foreach] cheap_copy crisis?

From: MB (mb2act_at_[hidden])
Date: 2005-12-02 20:47:45

Eric Niebler wrote:
> MB wrote:
> 
>>Hi,
>>
>>Under VC++7.1,
>>I was surprised that a class derived from 'boost::iterator_range'
>>was almost copied by BOOST_FOREACH.
>>('boost::noncopyable' works!)
>>
>>I think such a range that is noncopyable but derived from
>>iterator_range is valid, isn't it ...?
>>I think it could be dangerous if users can't find cheap_copy-customization.
>>
> Hrm, right. I feel some explanation is in order. The support for const 
> rvalue detection is not without runtime cost, and it's all to avoid 
> needless copying of expensive range-like objects such as STL containers. 
> But when the range is something small like a boost::iterator_range, 
> jumping through hoops of fire to avoid copying one is just silly. Hence 
> the cheap_copy hack: BOOST_FOREACH has an optimized code path for types 
> it knows (or believes to be) cheap to copy.
> 
> You have run afoul of the cheap_copy optimization. Rightly or wrongly, 
> it assumes types derived from boost::iterator_range are cheap to copy, 
> but that's not the case for you. There are two possible fixes, and they 
> both involve documenting the cheap_copy customization point (and 
> probably fixing it to use ADL like boost::begin()):
> 
> 1) By default, only apply the optimization to boost:iterator_range and 
> its ilk, NOT to types derived from them.
> 
> 2) Leave the code as it is, and force people in your situation to use 
> the cheap_copy customization point to disable the optimization.
> 
> I currently lean toward (1).

I will vote for (2) only because almost all my ranges are
cheap-copyable and derived from iterator_range :-)

But I don't know for sure.
I learnt from books to vote for (1).

Regards,
MB
http://p-stade.sourceforge.net/