GNAT'Object_Size

[Mats Weber <Mats.Weber@elca-matrix.ch>]

Stanley R. Allen wrote:

> While we are on the subject, it would be nice if the GNAT-defined
> 'Object_Size (a better name than the equivalent DEC-Ada-defined
> 'Machine_Size) attribute was universally adopted as well.  Those
> of us who must work at the bit level get frustrated when we cannot
> get at the actual number of bits that will be allocated for an
> entity, rather than the "minumum" given by <type>'Size.

And also generalize 'Object_Size so that it works on objects as well as types,
and returns the size actually allocated for an object, so that the following
code can be made to work:

generic
   type T (<>) is private;
procedure Receive (M : out T) is
   
   function Read (FD     : Integer;
                  Buffer : System.Address;
                  Count  : Integer) return Integer;

   pragma Import(C, Read);

   Length : Integer;

begin
   Length := Read(Socket,
                  M'Address,
                  M'Object_Size / System.Storage_Unit);
end;

Using 'Size in this situation is not portable because some compilers take it
to mean the size of the current value, not that of the object (which can be
different in the case of unconstrained types).