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: Peter Dimov (lists_at_[hidden])
Date: 2012-12-09 08:11:08

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.