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: Jeff Flinn (Jeffrey.Flinn_at_[hidden])
Date: 2012-12-10 09:05:49

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();
}

Jeff