Re: Mixing Ada and C++. Is a good idea?

[mheaney@ni.net (Matthew Heaney)]

In article <gwinn-1111972003010001@dh5055060.res.ray.com>,
gwinn@res.ray.com (Joe Gwinn) wrote:

>Beware variant records in OO.  Ada83 variant records have the well-known
>property that if anything in any record in the variant set changes,
>everybody who uses any member of that set must recompile.  In practice, in
>a project of any size, this means one recompiles the world just about
>every day.  

If you're recompiling the world every day, then something is wrong with the
design of the application, not with the language.

This phenomenon often occurs when a "common types" package is being used,
that contains a lot of low-cohesion type declarations that everyone with's. 
Add another type, and everyone who touches that package needs to be
recompiled.  You need to break the single, large, low-cohesion package up
into several smaller packages, that each contains a set a high-cohesion
types.  That way, when you add another type, only that corner of the
application that depends on that specific package needs to be recompiled.

If you're doing a message passaging (perhaps the common types package
declares a variant record containing all message types), then use the
streams facility, so a package only depends on the predefined streams
packages.  You might even consider deriving message types from a
Root_Message tagged type, so that message tags will be determined by the
compiler.  Better still is to use the distributed systems annex.

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Matthew Heaney
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