Re: By value-result vs by reference

andrewd@apanix.apana.org.au (Andrew Dunstan) writes:


>Barring the presence of an exception, the only way that you can "tell the
>difference" [between by-reference and value-in-value-out]
>in Ada is to write a routine that has a side effect directly
>modifying its parameters, i.e. by aliasing. If you write code like that
>you deserve what you get.


In Turing (and I am quite sure Euclid). Potential aliasing of "var"
parameters is prohibited, detectable, and detected by conforming
compilers. In fact, for large (i.e. nonscalar) value parameters, the same
prohibition is in effect. Thus large value parameters may be passed by
reference too.


In many ways this is a good thing. On the other hand, since aliasing and
potential aliasing are not the same thing, one is often thankful if ones
compiler does not treat this error as a fatal error.


> The language designers should not be required to
> cater for idiots writing bad code.


Good language designers recognize that even the best programmers are
occasionally idiotic. They also consider that maintenance is usually the
most costly part of the software life-cycle.


Theo Norvell
--

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