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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Why is the destructor called multiple times after I declare an
 object?
Date: Wed, 13 Jan 2016 20:31:53 -0500
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2016-01-13T20:31:53-05:00
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Jacob Sparre Andersen <sparre@nbi.dk> writes:

> Bob Duff <bobduff@theworld.com> writes:
>> Brian Drummond <brian@shapes.demon.co.uk> writes:
>
>> For nonlimited types, the compiler is free to use build-in-place if it
>> can prove that it's equivalent to return-by-copy.  It's often not
>> equivalent.  For example:
>>
>>     X := Init;
>>
>> What if Init raises an exception after partly modifying the result?
>> In that case X should not be modified, so the result object of Init
>> and X had better not be at the same address.
>
> But if:
>
> a) There is no exception handler between the assignment and leaving the
>    scope of X, build-in-place is okay (but impractical)?

Yes, I think that's OK.  I think it's also practical in some cases.

You also have to worry about the case where Init can see X.
X isn't supposed to (appear to be) changed until return from Init.

> b) The compiler can prove that no exceptions can occur after the first
>    modification of the result inside Init, build-in-place is okay?

Yes.  But it might be hard to prove that Storage_Error won't happen,
for example.

- Bob