Re: Object with zero bits

[Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de>]

Nick Roberts <nick.roberts@acm.org> wrote:
: Georg Bauhaus wrote:
: 
:> : You can declare a record that will have zero bits size. Is this what
:> : you want?
:> : 
:> :   type Frame_Mark is null record;
:> 
:> And maybe add a rep clause,
:> 
:>    type Nothing_At_All is null record;
:>    for Nothing_At_All'size use 0;
: 
: But many (perhaps most) compilers will make an object (or component) of
: type (subtype) Nothing_At_All at least size 1 (one bit).

An object of type Nothing_At_All can be 8 bits in size.
But Nothing_At_All as a record component seems to vanish,

  type Num is limited record
     virt: Nothing_At_All := initialize_Num(Num'access);
     n: Natural;
  end record;

  pragma assert(Num'Max_Size_in_Storage_Elements = 4);

With suitable rep clauses, Boolean discriminants don't seem to consume
space in some variant records, when the compiler can find out :-)