Re: [boost] safe-bool CRTP class

Subject: Re: [boost] safe-bool CRTP class
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2011-05-27 16:20:42

On Fri, May 27, 2011 at 1:13 PM, Steven Watanabe <watanabesj_at_[hidden]>wrote:

> On 05/27/2011 11:55 AM, Jeffrey Lee Hellrung, Jr. wrote:
> > On Fri, May 27, 2011 at 9:52 AM, Christian Holmquist
> > <c.holmquist_at_[hidden]>wrote:
> >
> >> On 27 May 2011 11:28, Stewart, Robert <Robert.Stewart_at_[hidden]> wrote:
> >>
> >>> Krzysztof Czainski wrote:
> >>>>
> >>>> Vladimir Batov prepared a safebool helper class as part of his
> >>>> Pimpl library.
> >>> [snip]
> >>>
> >>>
> >> There exists a safebool in already accepted Boost.Log:
> >>
> >>
> http://boost-log.svn.sourceforge.net/viewvc/boost-log/trunk/boost-log/boost/log/utility/explicit_operator_bool.hpp?revision=601&view=markup
> >>
> >
> > I'll chime in for my vote of something more along the lines of Andrey's
> > nullary macro design, except using a pointer-to-member-variable rather
> than
> > a pointer-to-member-function to address MSVC efficiency concerns (see
> prior
> > post by S.T. Lavavej).  I use something like the following, which I had
> also
> > picked up from Andrey in a past boost-dev list post.
> >
> > #define BOOST_EXPLICIT_BOOL() \
> > private: \
> >     struct _boost_explicit_bool_struct \
> >     { int _dummy; int member; }; \
> >     typedef int (_boost_explicit_bool_struct::*_boost_bool_type); \
> > public: \
> >     operator _boost_bool_type() const { return !*this ? 0 :
> > &_boost_explicit_bool_struct::member; }
> >
> > When BOOST_NO_EXPLICIT_CONVERSION_OPERATORS is not defined, then the
> above
> > macro is defined to just declare an explicit bool conversion, so it
> amounts
> > to the same framework you would use in C++0x.
> >
> > I prefer a macro solution over a CRTP solution because 1) it retains
> > POD'ness (in C++03, at least; I believe the definition of POD has been
> > extended in C++0x...?); and 2) the macro appears in the same place one
> would
> > otherwise declare a bool conversion operator.
> >
> > I also like the solution to use operator! by default in the
> implementation
> > of the explicit bool conversion operator; perhaps a second macro could be
> > provided to allow one to specify the specific expression (e.g., "!!*this"
> or
> > "as_bool()") one should use.
> >
>
> What about a macro that can be used like
>
> BOOST_EXPLICIT_BOOL_OPERATOR() const {
>    return ...;
> }
>

Are you thinking something along the lines of

#define BOOST_EXPLICIT_BOOL_OPERATOR() \
[...define a safe-bool operator that calls, e.g., _boost_as_bool() to
determine true'ness...] \
bool _boost_as_bool()

?

- Jeff