emery@MBUNIX.MITRE.ORG (Emery) (12/11/89)
Armin points out that it is perfectly legal to do conditional
compilation of statements based on some (constant) boolean value.
This is something I'd expect a good compiler to do, and a technique
that I have used (sending prayers to the God of Compilers that my
compiler was "good").
Where this approach fails, though, is in non-statements such as type
declarations. It would be nice to have support for this:
type foo_bar is
record
x : integer;
#ifdef VAX
y : float_64;
#elsif IBM_PC_WITH_8087
y : IEEE_float;
#endif
end record;
I suspect that the very clever person could use variant records for
this example, but I also suspect that it wouldn't take much to come up
with type definitions where a variant record approach wouldn't work.
Using these ("constant type") variant records would probably make the
program harder to read, because it wouldn't be clear to the maintainer
if this was a variant record object (or subtype) based on some 'real
variant) or based on some conditional compilation feature. Objects
are often widely separated from their declaration, compared to
(optimizable) conditional compilation statements.
dave
emery@aries.mitre.orgmunck@linus.UUCP (Robert Munck) (12/12/89)
In article <8912110202.AA11909@mbunix.mitre.org> emery@MBUNIX.MITRE.ORG (Emery) writes: >... (discusses conditional compiliation of executable code)... > >Where this approach fails, though, is in non-statements such as type >declarations. It would be nice to have support for this: > record >#ifdef VAX > y : float_64; >#elsif IBM_PC_WITH_8087 > y : IEEE_float; >#endif > >... but I also suspect that it wouldn't take much to come up >with type definitions where a variant record approach wouldn't work. I'm not so sure about this anymore. I have a feeling that "type definitions where a variant record approach wouldn't work" may well be the equivalent of "programs where structured coding doesn't work." We oldtimers are all used to playing fast and loose with the source text with preprocessor statements, including such horrors as "manufacturing" keywords from conditionals and compile-time variables. Maybe, just maybe, that isn't such a good idea, and the good software engineering practice would be to stick to the language definition, variant records and all. I have only one data-point, a small OS with a fair number of "sysgen" variables and choices. I found it fairly easy and "satisfactory to my esthetic sense" to do everything entirely within the language.