Re: exception behaviour in containers

From:
David Abrahams <dave@boost-consulting.com>
Newsgroups:
comp.std.c++
Date:
Mon, 15 Oct 2007 13:16:23 CST
Message-ID:
<87myuktjjx.fsf@grogan.peloton>
on Mon Oct 01 2007, Daniel Kr??gler <daniel.kruegler-AT-googlemail.com> wrote:

On 1 Okt., 19:42, d...@clovermail.net (Daniel) wrote:

[note to moderators: I tried sending this to the newsgroup about 48 hours ago
but I can't see it so I am trying again to the email address]

I can't see any descriptions/guarantees of what should happen if there is an
exception in certain container member functions. I am specifically interested
in copy constructor, assignment operator, and insertion. It seems there are a
few possibilities that could happen:

* unwind any changes and leave the container in the state previous to the
function call
* leave the operation partially complete, some elements copied
* abort leaving the container internal data structure corrupted


They are there.

As recently explained by David Abrahams in the thread " Standard
library exception specifications might be lacking" found at

http://tinyurl.com/2vqroj

the current standard finished this part a little bit hasty due to
time- limits and one of the out-comes of the thread was that at
least some points are still open (begin(), iterator::operator+,
etc).


I don't think that's an accurate characterization. We did provide
completely documented guarantees for all the operations in question.
Time limits prevented us from providing stronger guarantees than we did.

Regrettably it is no-where explicitly written, but the intend
of the STL seems to be to provide at least the basic safety
guarantee.


It doesn't need to be explicitly written. Nowhere does the standard
permit the standard library components to violate invariants.

I base this assertion on more recent attempts to clarify
the exception behaviour of the standard library in upcoming
multi-threading C++ as described in

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2410.html


That paper has almost no relevance to the question, AFAICT.

I don't think that the above mentioned copy-c'tor is relevant for the
discussion, because a failing c'tor will prevent the existence of a
to-be-constructed object and thus there is not much to corrupt (or
do you possibly mean the source??).


The copy ctors with const& rhs are not allowed to modify the source.
That leaves one standard template, auto_ptr, where such considerations
are relevant.

In [container.requirements]/10+11 of the recent draft there are
given general statements on insert for the single-insertion step (so
this is not limited to unordered associative containers).


That's not a new addition.

I think it was considered as too expensive to provide the strong
guarantee for multi-inserts and if needed is easy to write.
Further-on several containers to give more precise guarantees, e.g.
in [deque.modifiers] or [vector.modifiers]


Correct.

--
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com

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