Re: [boost] [optional] memory use for optional refs and ptrs

Subject: Re: [boost] [optional] memory use for optional refs and ptrs
From: Larry Evans (cppljevans_at_[hidden])
Date: 2010-10-11 07:16:27

On 10/07/10 07:31, Rutger ter Borg wrote:
> On 2010-10-07 13:37, Larry Evans wrote:
> 
>> [snip]
>>
>> A more complete facility use is here:
>>
>>
>> https://svn.boost.org/trac/boost/browser/sandbox/variadic_templates/libs/composite_storage/sandbox/layout_all.cpp
>>
[snip]
>> which shows the padding after the char is 3 in order
>> to make the alignment of the int at 4.
>>
>> HTH.
>>
>> -Larry
> 
> Thanks,
> 

You're welcome.

The reusable part of the code has been refactored into:

  http://svn.boost.org/svn/boost/sandbox/variadic_templates
    /boost/composite_storage/utility
    /show_layout_all_of_aligned.hpp

so that it on different struct's rather than just the struct's
in the current example driver.

The output produced on the example struct's in:

  https://svn.boost.org/trac/boost/browser/sandbox/variadic_templates
    /libs/composite_storage/sandbox/layout_all.1.cpp

is now in tabular format:

A_type
===========
***components***
----------------
num size align sum_size
  1    4     4        4
  2    8     8       12
  3    8     8       20
  4    8     8       28
***composite***
---------------
num size align pad sum_pad offset
  5   32     8   0       0      _
  4   32     8   0       0     24
  3   24     8   0       0     16
  2   16     8   4       4      8
  1    4     4   0       4      0
  0    0     1   _       _      _
A_bools_type
===========
***components***
----------------
num size align sum_size
  1    4     4        4
  2    8     8       12
  3    8     8       20
  4    8     8       28
  5    1     1       29
  6    1     1       30
  7    1     1       31
  8    1     1       32
***composite***
---------------
num size align pad sum_pad offset
  9   40     8   4       4      _
  8   36     8   0       4     35
  7   35     8   0       4     34
  6   34     8   0       4     33
  5   33     8   0       4     32
  4   32     8   0       4     24
  3   24     8   0       4     16
  2   16     8   4       8      8
  1    4     4   0       8      0
  0    0     1   _       _      _
bools_A_type
===========
***components***
----------------
num size align sum_size
  1    1     1        1
  2    1     1        2
  3    1     1        3
  4    1     1        4
  5    4     4        8
  6    8     8       16
  7    8     8       24
  8    8     8       32
***composite***
---------------
num size align pad sum_pad offset
  9   32     8   0       0      _
  8   32     8   0       0     24
  7   24     8   0       0     16
  6   16     8   0       0      8
  5    8     4   0       0      4
  4    4     1   0       0      3
  3    3     1   0       0      2
  2    2     1   0       0      1
  1    1     1   0       0      0
  0    0     1   _       _      _

which shows the padding between individual
fields and the sum of the padding.  This shows
why putting the bools first (bools_A_type)
saves space.  As mentioned before the 4 bools
are placed in the padding present in the
padding required by A_type; hence,
sizeof(A_type) == sizeof(bools_A_type).

Hope the tabular output makes it easier
to see the reason for space usage.

-Larry