Re: Collective response to := messages

[billwolf@hubcap.clemson.edu (William Thomas Wolfe,2847,)]

From article <10917@ulysses.homer.nj.att.com>, by dsr@hector.UUCP (David S. Rosenblum):
> In article <755@marvin.cme-durer.ARPA> leake@cme-durer.ARPA (Stephe Leake) writes:
>>
>>There seems to be a point of confusion concerning overloading of ":=".
>>Some people assume that the (overloaded) semantics of ":=" apply to
>>parameter passing. This seems to be a reasonable assumption, and I
>>think the current Ada parameter passing semantics are equivalent to
>>the current Ada assignment semantics.
> 
> They are not the least bit equivalent, for several reasons.  The most
> obvious reason is that an implementation is free to pass composite
> parameters by reference, although no implementation can "assign by
> reference". 

    Sure, for "in out" mode.  But for modes "in" and "out", failure to
    use the ADT's assignment procedure causes major problems:

      "out" mode:  Programmer assigns the parameter a value, and the
                   value is copied back to the actual parameter.  
                   Unfortunately, the portions of the actual parameter
                   which were formerly accessible by pointers are now
                   uncollectable garbage.

       "in" mode:  Programmer assumes that this is a "pass by value"
                   and makes modifications.  Since ADT contains pointers,
                   modification echoes through to the actual parameter.
                   Programmer could also invoke DESTROY, blowing away
                   major portions of the actual parameter.