Re: [boost] [typeof] void return

From: Peder Holt (peder.holt_at_[hidden])
Date: 2006-01-04 02:46:34

On 1/3/06, Alexander Nasonov <alnsn_at_[hidden]> wrote:
> There are two problems with void returns.
>
> 1. A couple of examples in documentation are inaccurate:
>     main returns void. The fix is
>
> $ cd $BOOST_ROOT/libs/typeof/doc
> $ sed 's/void main/int main/' typeof.qbk
>
> 2. BOOST_TYPEOF can't be applied to an expression returning void.
>
> void foo() {}
> int main()
> {
>      BOOST_TYPEOF(foo());
>      // Error: ensure_obj doesn't accept void as an argument.
> }
>
>
> The patch below fixes this problem.
> It's built around comma operator:
>
> struct void_ {};
> template<class T> T& operator,(T const&, void_);

How will this work if someone implements their own operator,?
template<class T> T& operator,(const my_type&,const T&);

if expr equals my_type, wouldn't this break typeof for my_type?

Peder

>
> template<class T> T& ensure_obj(const T&);
> void ensure_obj(void_);
>
> Return type of an expression ((expr), void_()) is
>   T&, if typeof(expr) == T && T != void
>   void_, if typeof(expr) == void
>
> I ran tests on gcc 3.4.4 under FreeBSD. All tests passed. I haven't
> tried to analyze the problem on MSVC.
>
> Let me know if fixes are fine and I'll commit them.
>
> $ cvs diff -u boost/typeof/typeof.hpp
> alnsn_at_[hidden]'s password:
> Index: boost/typeof/typeof.hpp
> ===================================================================
> RCS file: /cvsroot/boost/boost/boost/typeof/typeof.hpp,v
> retrieving revision 1.3
> diff -u -r1.3 typeof.hpp
> --- boost/typeof/typeof.hpp     28 Sep 2005 03:47:05 -0000      1.3
> +++ boost/typeof/typeof.hpp     3 Jan 2006 21:51:47 -0000
> @@ -128,10 +128,15 @@
>   #   ifndef MSVC_TYPEOF_HACK
>
>           namespace boost { namespace type_of {
> +            struct void_ {};
> +            template<class T> T& operator,(T const&, void_);
>               template<class T> T& ensure_obj(const T&);
> +            void ensure_obj(void_);
>           }}
>
> -#       define BOOST_TYPEOF(expr)
> BOOST_TYPEOF_KEYWORD(boost::type_of::ensure_obj(expr))
> +#       define BOOST_TYPEOF(expr) BOOST_TYPEOF_KEYWORD(
>       \
> +            boost::type_of::ensure_obj( ((expr),
> boost::type_of::void_()) ) \
> +        )
>   #          define BOOST_TYPEOF_TPL BOOST_TYPEOF
>   #   endif
>   #      define BOOST_TYPEOF_REGISTER_TYPE(x)
>
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