Re: The "()" operator revisited.

["Warren W. Gay VE3WWG" <warren@ve3wwg.tk>]

Mark A. Biggar wrote:
> Robert A Duff wrote:
>> "Warren W. Gay VE3WWG" <warren@ve3wwg.tk> writes:
>>> Robert A Duff wrote:
>>>> "Warren W. Gay VE3WWG" <warren@ve3wwg.tk> writes:
>>>>
>>>>> Was operator assignment ever considered for Ada? If it was, and shot
>>>>> down, what were the primary reasons against it?
>>>>
>>>> See AARM-7.6(17.a).
>>>> - Bob
>>>
...
>> Sorry, I didn't realize you were talking about Ada 83.  Why can't the
>> user redefine ":=" in Ada 83?  I don't know, but I guess the designers
>> didn't like users redefining stuff like that.  Same reason the user
>> can't redefine "in" or aggregate notation or literal notation or array
>> indexing notation, I guess.  It seems inconsistent with the fact that
>> you *can* redefine "=" and "+" and so forth.
...
>> The C++ way has some advantages over the Ada way (where the user's
>> control is split between Finalize and Adjust) because it's sometimes
>> convenient to get your hands on both sides of the assignment at the same
>> time.
> 
> There are other issues as well.  Ada has several things that look like
> assigmment but are subtly different: Variable initialization, function 
> return, by-value parameter binding, etc.  It was decided that allowing
> a user to redefine assignment piecewise (initialize, adjust, finalize)
> would allow the compiler to compose the various assignment like 
> operations out of those pieces and other things like bit-wise memory
> copy without requiring the user to define all those different variants.

OK, I am beginning to see where the issues lie. I suppose one
extreme example involves variant records, where the size of the
variant must be known in advance. A strict bit-by-bit copy followed
by tweaking makes perfect sense to me in this scenario. Although,
I suppose an implementation could assign maximum storage and
potentially resize the object after the ":=" procedure was done.

But it looks like there are some other issues involved in Ada that
don't come up in a C++ context. Thanks for the explanations.

Warren.

-- 
Warren W. Gay VE3WWG
http://ve3wwg.tk