Re: [boost] [xint] Third release is ready, requesting preliminary review

Subject: Re: [boost] [xint] Third release is ready, requesting preliminary review
From: Christopher Jefferson (chris_at_[hidden])
Date: 2010-05-03 08:40:18

On 3 May 2010, at 13:28, Stewart, Robert wrote:

> Christopher Jefferson wrote:
>> On 3 May 2010, at 12:39, Stewart, Robert wrote:
>>> Chad Nelson wrote:
>>>> 
>>>> algorithms that have to start with a copy of one or more of the
>>>> passed-in parameters, but then need to modify it/them. The division
>>>> algorithm requires this, just as one example off the top
>>>> of my head.
>>> 
>>> If your algorithm requires a copy, then COW should make it
>>> slower because it will first manipulate the reference count
>>> and because you must query in each mutating operation whether
>>> to make a unique copy.
>> 
>> But, you save the cost of copying a large object. COW has
>> many disadvantages, but how can you claim it is slower when
>> you require a copy?
> 
> Did you miss the "but then need to modify it/them" part?  The only reason for the copy was to have a mutable copy.  Thus, the COW overhead was useless in that example.

Apologises, I did indeed not read the message closely enough.

My experience from COW std::string implementation is that with care, rvalue references and copy elising, you can get away with almost no unnecessary copies. In particular, in C++0x having the compiler always move local variables in return statements is a big win. How much of this gain you would get in a plain C++03 compiler, with boost::move but without users annotating any moves or rvalues to their code, I don't know.

Chris