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@cvs.sourceforge.net'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)