NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/std/c++/1124

Internet Message Format | 1996-08-06 | 3.0 KB

From: "Wil Evers" <[e-mail]>
Message-ID: <[e-mail]>
X-Original-Date: Wed, 17 Apr 96 10:32:06 WET
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Date: 17 Apr 96 10:00:22 GMT
Approved: [e-mail]
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Newsgroups: comp.std.c++
Subject: Re: exceptions in std lib
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In article <[e-mail]> Nathan Myers <[e-mail]>
writes:

> Before you criticize this, consider what you would expect vector<>
> to do when it is copying a block of elements from one memory region
> to another, or deleting a block of elements, and an exception happens
> in the middle.  Do you stop there, half-done?  Do you copy or delete
> the rest of the elements and leave a hole, then rethrow?  (What if you
> get another exception while doing the rest?)
>
> If you propagate the exception, member functions of the vector
> (including the destructor) will probably be called later.  What will
> they find?
>
> This is a long way of asking: What do you mean by `required to be
> exception-safe'?  If you can define it, then probably the standard
> library ought to meet the definition.

It seems to me that Harald Mueller's article in the January '96 issue of
The C++ Report could be a big conceptual help here. Basically, after an
operation on an object has reported an exception, one of three things can
be true:

1. The object is in the state it was before the operation that reported the
exception was tried ('throw and keep'). IMHO this should be the default for
standard library components.

2. Futher operations on the object do not make sense, but it is still
possible to destroy it ('throw and throw away'). For some operations, it
may be impossible or practically infeasible to impose rule (1). Here, the
standard could require the operation's documentation to explicitly state
that this is the case. (Since the only sensible thing we can do with such
an object is to destroy it, the standard could also require that a standard
library object in such a state must throw an exception on every attempted
operation until it is destroyed.)

3. The object is in such an unstable state that it is not even possible to
destroy it. This should never happen. (Note: this implies that exceptions
should never escape from destructors.)

Of course, 'exception unsafety' is a cascading problem. It wouldn't make
much sense to require a template taking an argument of a type T to be
throw-and-keep if some of T's operations it depends on are
throw-and-throw-away.

- Wil
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