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

Subject: Re: [boost] [cpo-proposal] presentation of the idea
From: Rob Stewart (robertstewart_at_[hidden])
Date: 2013-08-20 05:04:50

On Aug 19, 2013, at 9:44 AM, Thorsten Ottosen <thorsten.ottosen_at_[hidden]> wrote:

> 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 :(

Reallocation causes a bigger problem...

>> 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.

Sure, the new objects would be laid out in memory the same after a reallocation, but...

>> 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>.

Here's the problem. Replicating the bits of and object isn't the same as copying it. Many classes store a this pointer or rely on a data member address as a unique key, etc. Copy constructors must run.

If the container manages the reallocations by noticing when it is about to occur, creates a new vector with greater capacity, clones each element into the new vector, destroys the elements in the old vector, then swaps the two vectors, it can work.

I should think a deque would be better as it doesn't require all of that work.

___
Rob

(Sent from my portable computation engine)