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Subject: Re: [boost] [function] function wrapping with no exceptionsafetyguarantee
From: Edward Diener (eldiener_at_[hidden])
Date: 2010-10-21 18:13:12

On 10/21/2010 6:05 PM, Daniel Walker wrote:
> On Thu, Oct 21, 2010 at 2:25 AM, Doug Gregor<doug.gregor_at_[hidden]> wrote:
>> On Wed, Oct 20, 2010 at 1:51 PM, Daniel Walker
>> <daniel.j.walker_at_[hidden]> wrote:
>>> On Tue, Oct 19, 2010 at 4:56 AM, David Abrahams<dave_at_[hidden]> wrote:
>>>> At Tue, 19 Oct 2010 01:06:00 -0700,
>>>> Emil Dotchevski wrote:
>>>>> On Tue, Oct 19, 2010 at 12:34 AM, Domagoj Saric<dsaritz_at_[hidden]> wrote:
>>>>>> "Emil Dotchevski"<emil_at_[hidden]> wrote in message
>>>>>>> ... at worst they'd be mad that you've used
>>>>>>> Boost (that's common in games, for example.)
>>>>>> Shall we disregard all those cases (of Boost rejection) as irrational rants
>>>>>> (as admittedly they often are, be it of the 'corporate policy' type or of
>>>>>> the Linus Torvalds type) or shall it be admitted that after all, sometimes,
>>>>>> they actually are based on real objections (that Boost, or parts of it, made
>>>>>> some not-so-happy efficiency compromising choices)...?
>>>>> You can't talk about Boost efficiency in general. As difficult as it
>>>>> is to pull apart, Boost contains individual components. Are we talking
>>>>> about the efficiency of Boost Function then? I'm sure if someone
>>>>> manages to speed it up, many people on this mailing list (not to
>>>>> mention the folks who are implementing std::function) would be very
>>>>> interested to see how it can be done.
>>>> I think we already know one way: we can easily get rid of the separate
>>>> empty() check by making sure empty boost::functions all invoke a
>>>> function that throws bad_function_call.
>>> After taking a closer look at this idea, I don't think it is possible
>>> to do this without either changing boost::function's semantics or
>>> incurring other runtime expenses.
>>> First of all, boost::function can be in an empty state for two
>>> reasons: either it has not been assigned a target function or there
>>> has been a problem with the internal target management system. Both of
>>> these conditions are tested simultaneously by the current empty check
>>> in operator(). If we get rid of the empty check, we will no longer be
>>> checking that the target management system is in working order and
>>> able to dispatch function calls (or more specifically, that
>>> boost::function's internal vtable pointer is non-null). If the target
>>> management system is not in an usable state, then we cannot dispatch a
>>> default "empty" function that throws bad_function_call. So, if we get
>>> rid of the current empty check but retain the current target
>>> management system, we open the possibility that boost::function could
>>> be in an empty state but would not throw when invoked, which would be
>>> a change in semantics.
>> The latter "state" isn't a real state; it only exists if there's a bug
>> in boost::function, and we don't design around bugs.
> Right. I don't mean the management system has a bug, I mean that it
> has encountered a problem, for example, cloning the target. So,
> boost::function could be empty because it has never been assigned a
> target or because the most recent attempt to assign a target failed...
> or because it was cleared by the user calling clear(). Are those all
> of the scenarios that can lead to an empty boost::function?

I don't mean to question the design of boost::function but wouldn't the
inability of cloning a target, or assign a target, be a problem which
should lead to an exception being thrown ?

I would assume that boost::function not having a target would normally
only occur if no target had been set or if the user removed a target
which had been set.

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