[llvm] r206372 - [Allocator] Make BumpPtrAllocator movable and move assignable.

[David Blaikie]

On Fri, Apr 18, 2014 at 1:43 PM, Chandler Carruth <chandlerc at gmail.com> wrote:
>
> On Thu, Apr 17, 2014 at 7:04 AM, Duncan Exon Smith <dexonsmith at apple.com>
> wrote:
>>
>> The existence of a move constructor without a corresponding explicit copy
>> constructor seems sufficiently explicit of a marker for "move only" to me.
>>
>>
>> I think it looks more like an accident, not a marker.
>>
>> I prefer Stroustrup's advice [1] on this.  It's not practical to follow
>> all the time in this project because of MSVC support (no sense explicitly
>> defining a default definition move constructor by hand if it's not going to
>> be used anyway), but the style still makes sense to me when it's possible.
>> It's cheap, and makes it clear that the omission is intentional.
>>
>> [1]: http://www.stroustrup.com/C++11FAQ.html#default2
>>
>> Do you disagree strongly?
>
>
> I dunno about strongly, but it feels all wrong to me.
>
> It feels like needless boilerplate, and I have a real allergy to
> boilerplate. I suppose my real fear is that if programmers were to follow
> this pattern consistently it would become a burden. At that point, they will
> just stamp out the boiler plate using a macro or a some template in an IDE
> or whatever, and they still won't think about it so we won't have solved
> anything.
>
> My preference is rule of zero, and minimally adding the routines you need.
> This trusts the language to always DTRT when you add the subset, but I think
> the language gets this right here.
>
> /me continues to ponder.

FWIW I agree with Chandler here - by only implementing the minimal set
of members needed and trusting the language/compiler to do the right
thing we're in a better place.

Boilerplate is not not just visual noise for readers but can actually
hurt us: should a type one day become copyable (when it was previously
movable) then all types that have it as a member would need to be
updated.

Though I agree that it takes some effort to kick the "Rule of 3(5)"
habit and trust the compiler - but we can all help each other to get
there.

- David