Re: [boost] [foreach] cheap_copy crisis?

From: Eric Niebler (eric_at_[hidden])
Date: 2005-12-02 18:38:42

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).

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com