Re: [boost] safe-bool CRTP class

Subject: Re: [boost] safe-bool CRTP class
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2011-05-27 16:13:06

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 ...;
}

In Christ,
Steven Watanabe