Re: [boost] interest in structure of arrays container?

Subject: Re: [boost] interest in structure of arrays container?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2016-10-20 04:55:53

On 10/19/2016 11:59 PM, Michael Marcin wrote:
 > On 10/19/2016 7:32 AM, Larry Evans wrote:
 >>> Michael, the attached is an outline of how to do it.
 >>> As noted in the comments, a lot of member functions still need
 >>> implementation; however, the offset calculations I think are
 >>> correct.
 >>>
 >> Code has been updated to implement most of the "essentials" of
 >> soa_block.  You can now call begin<Index> to get the begin
 >> iterator for the Index'th vector.
 >>
 >> The code now is on github at:
 >>
 >> https://github.com/cppljevans/soa/blob/master/soa_block.cpp
 >>
 >
 > Still digesting this

You may be having trouble with digesting:

   soa_impl<...>::get_self
   soa_impl<...>::get_vec

and also digesting:

     ~soa_impl()
       {
         using swallow = int[]; // guaranties left to right order
         (void)swallow{0, (void(this->template destruct<Indices>()), 0)...};

both techniques I copied from elsewhere (I can't remember where).
If so, I could explain a bit.  If it's something else, I'd
be happy to try and explain that.

 > but one small tidbit I stood out for me
 >
 > //The c++ standard, IIRC, says all alignments are power of 2.
 >
 > I didn't actually know this was a requirement.
 >
 > "Every alignment value shall be a non-negative integral power of two."
 > § 3.11/4 http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/n4606.pdf
 >
 > Thanks!

You're welcome.

One change I made to the vec_offsets function could lead to
a problem in some very special application.  You might think
that you could, instead of:

       , my_storage(new char[my_offsets.back()])

in the soa_block CTOR initialization list, you wanted to
create a vector of vectors, or matrx, of soa's.  Then you
might think you could:

     soa_impl(std::size_t a_row_size, std::size_t a_col_size)
       : my_row_size(a_row_size)
       , my_col_size(a_col_size)
       , my_offsets(vec_offsets<Ts...>(my_row_size))
       , my_storage(new char[my_offsets.back()*my_col_size])
       {
       ...
       }

However, you couldn't because the 2nd column of the matrix
*might* not have the correct alignment.  That's because, I
changed the vec_offsets function by replacing, in the
vec_offsets function, this:

     result[sizeTs]=nxt_offset( result[sizeTs-1], 
sizes[sizeTs-1]*vec_size, max_align);

with this:

     result[sizeTs]=result[sizeTs-1]+sizes[sizeTs-1]*vec_size;

My justification for doing that was I assumed nobody would
want to do that; however, who knows.  Also, if I left in the
original, and there was no need for it, then people
reviewing the code would wonder why I put it in :(

I'll try to find some comments to put in the vec_offsets
function to warn people of this unlikely problem.

HTH,

Larry