Re: [boost] Improving the assignment operators of various Boosttypes

Subject: Re: [boost] Improving the assignment operators of various Boosttypes
From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2008-09-10 15:45:59

On Wed, Sep 10, 2008 at 9:30 PM, David Abrahams <dave_at_[hidden]> wrote:
>
> on Wed Sep 10 2008, "Giovanni Piero Deretta" <gpderetta-AT-gmail.com> wrote:
>
>> If you do not want to deal with move emulation (which I've found very
>> brittle in complex expressions),
>
> Do you mean the old-style move emulation that relied on T(T&) copy ctors
> (known to be brittle) or does this assessment apply also to the
> adobe-style emulation
> (http://stlab.adobe.com/group__move__related.html)?  If so, could you
> explain in more detail?

The former; I have yet to try the latter. I got interested in the
boost::move because it supported move only types (which IMHO are the
really interesting application of move semantics). Too bad that
doesn't seems to be a robust solution in C++0x.

>
>> a simple way to to gain the advantage
>> of T::operator=(T rhs) even when assigning from lvalues is something
>> like:
>>
>>    template<class T> T destructive_copy(T& x) {
>>       using std::swap;
>>       T result;
>>       swap(result, x);
>>       return x;
>>    }
>
> This only works when T is both default constructible and has an
> optimized swap.  The latter is detectable, roughly speaking, so the
> function could be altered to handle it, but the former is not, which it
> seems to me makes destructive_copy not very useful in generic code.
>
> ...
>
>> For types which do not have an optimized swap is suboptimal, so some
>> sfinae trick on is_swappable might be needed. Ah, of course it
>> requires T to be DefaultConstructible, and most importantly
>> CopyConstructible so it doesn't handle move only types.
>
> That's a lot of caveats.

I think that the 'adobe move' has the same requirements. OTOH,
destructive_copy requires no specific support from the 'moved' type: a
custom swap and the 'smart' operator= are useful on their own, so
adding destructive_copy to one's set of tools requires very little
extra baggage.

-- 
gpd