Re: Ada Protected Object Tutorial #1

[Robert Dewar <robert_dewar@my-deja.com>]

In article
<Pine.A41.3.96-heb-2.07.991226141254.25330A-100000@pluto.mscc.hu
ji.ac.il>,
> This very interesting article tries to explain why it
> is almost always better to use protected types, and create
> less tasks. It gives detailed examples of many tasking idiom,
> so it is also helpful as a guide to Ada tasking.


I disagree with this conclusion. Indeed my view is that it is
almost always better to use an intermediate task than a
protected object unless you specifically need the low level
efficiency that protected objects can provide in some
implementations.

Why? Because protected objects in Ada have relatively nasty low
level semantics that do not compose properly, because of the
restrictions on potentially blocking operations (these
restrictions are totally unnecessary from a semantic point
of view, they are there just to allow a more efficient
implementation on certain limited kinds of architectures).

This means that if you use protected objects you have to operate
at a lower level of abstraction.

Yes, it is always reasonable to lower the level of abstraction
in the interests of gaining efficiency, but you only do this if
you need to. The fact that when you write a protected body, you
have to be aware of the *implementation* of any subprogram that
you call, to be sure that it has no blocking operations, is
a serious disadvantage of using PT's.


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