Re: Ada95 Streams Question

[dewar@cs.nyu.edu (Robert Dewar)]

Sam said

"To be general, Ada compilers are free to use any representation of
streams. One may well choose raw conversions (as GNAT does) while
another one will adopt XDR encoding."

That's correct, as we see from 13.13.2

9   For elementary types, the representation in terms of stream elements is
implementation defined.  For composite types, the Write or Read attribute for
each component is called in a canonical order.  The canonical order of
components is last dimension varying fastest for an array, and positional
aggregate order for a record.  Bounds are not included in the stream if T is
an array type.  If T is a discriminated type, discriminants are included only
if they have defaults.  If T is a tagged type, the tag is not included.


GNAT by default follows the implementation advice in paragaph 17

17   If a stream element is the same size as a storage element, then the
normal in-memory representation should be used by Read and Write for scalar
objects.  Otherwise, Read and Write should use the smallest number of stream
elements needed to represent all values in the base range of the scalar type.


However, this is easily changed, the library package System.Stream_Attributes
(in files s-stratt.ads/adb) comletely define the implementation choice for
representation of elementary types, and this unit is easily replaced, e.g.
by one that uses XDR or some other canonical representation.