Re: [boost] [smart_ptr][thread][test] breaking change on shared_ptr operator bool.

Subject: Re: [boost] [smart_ptr][thread][test] breaking change on shared_ptr operator bool.
From: Gaetano Mendola (mendola_at_[hidden])
Date: 2012-12-11 18:40:54

On 10/12/2012 15.05, Jeff Flinn wrote:
> On 12/9/2012 8:11 AM, Peter Dimov wrote:
>> Vicente J. Botet Escriba wrote:
>>> Le 08/12/12 15:20, Peter Dimov a écrit :
>>> > Vicente J. Botet Escriba wrote:
>>> >> Hi,
>>> >>
>>> >> https://svn.boost.org/trac/boost/changeset/81780 " Use explicit >>
>>> operator bool when available; ..." introduces a breaking change on >>
>>> shared_ptr in Boost.Thread and Boost.Test at least.
>>> >
>>> > What code does it break?
>>>
>>> Code like
>>>
>>> bool thread::joinable() const BOOST_NOEXCEPT
>>> {
>>>    return get_thread_info(); // COMPILE ERROR
>>> }
>>
>> Yes, this is a bit unfortunate side effect of the language specification
>> of "explicit operator bool". It does not get invoked in return
>> statements such as the one above, and it does not get invoked when
>> calling a function that takes a bool. The second case makes sense
>> pragmatically, but the first doesn't. Still, the principle behind the
>> specification is that an explicit operator bool is only invoked when a
>> language construct such as if, while, or operator! needs a bool, and the
>> above case is not one of those situations.
>>
>> Since all of the standard library in C++11, including std::shared_ptr,
>> has been specified to use explicit operator bool, and since the
>> committee did not display any enthusiasm for changing this status quo in
>> some way so that the code above could continue to work, I think that we
>> better get used to it. :-)
>>
>>> which I have replaced by
>>>
>>> bool thread::joinable() const BOOST_NOEXCEPT
>>> {
>>>    return get_thread_info()!=0;
>>> }
>>>
>>> Is there a better way?
>>
>> I think that this way is fine.
>
> FWIW I use:
>
> bool thread::joinable() const BOOST_NOEXCEPT
> {
>      return !!get_thread_info();
> }

That's horrible.

Regards
Gaetano Mendola