Re: [boost] [cpo-proposal] presentation of the idea

Subject: Re: [boost] [cpo-proposal] presentation of the idea
From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2013-08-19 09:44:35

On 19-08-2013 15:10, Larry Evans wrote:
> On 08/19/13 07:54, Larry Evans wrote:
>> On 08/19/13 06:59, Thorsten Ottosen wrote:

>>>    cont.push_back<Derived>( x, y );
>>>
>>
>> Using a templated push_back, IIUC, would require some sort of
>> container, such as std::vector<char> cont, which contains contigous
>> storage, and the push_back should find the next location, i_aligned,
>> in std::vector<char> which is at alignment, alignof(Derived).
>> and would then push_back i_aligned chars, then further push_back
>> sizeof(Derived) char's, the new inplace at cont.begin()+i_aligned.
>>
>> Does that make sense?
>>
> OOPS.  If all those push_back's of a char causes the vector to be
> resized in placed in a different location, then wouldn't that require
> all the realignments for each element to be recalculated :(
>
> Hmmm.  Not if &cont[0] is stored at a max aligned location,
> which I assume it would be since the storage would be created
> on the heap with new[] which, according to the c++ standard
> ( can't remember which page or section), returns a maximally
> aligned storage location.

Exactly. The relative locations should stay the same, only the start 
address would change.

> And since the start of cont
> is maximally aligned, any offsets in that cont with a given
> alignment would have the same alignment when moved, IIUC.

Yes, at this level it is nice to take advantage of the memcopying built 
into vector<char>.

I guess the above means that the idea of letting push_back return the 
address of the object is not going to work /unless no reallocation 
happens/. This may be too fragile, so maybe we should embed a singly 
linked list directly together with the DerivedX instances and update it 
on copying. This seems to contradict the use of memcpy. Hm.

Maybe we can avoid storing the linked list al together. Forward 
iteration would find the next element by asking the current element by 
its size and by computing i_aligned (somehow). Maybe that's the way to go.

regards

-Thorsten