[boost] [typeof] void return

From: Alexander Nasonov (alnsn_at_[hidden])
Date: 2006-01-03 17:21:17

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_);

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)