Attempt to solve issue #10 (#24)

* Attempt to solve issue #10

In boost::current_exception(), after all supported types of exception have
been checked for, a boost::unknown_exception is stored in the
boost::exception_ptr as a last resort to signal that an exception was indeed
stored, but that the actual type and value of the exception was lost.

Now, in case C++11 std::current_exception() is supported, the
std::exception_ptr result of calling std::current_exception() is what
gets stored inside the boost::exception_ptr. Later, inside
boost::rethrow_exception, the std::exception_ptr is retrieved from
the boost::exception_ptr and whatever exception it stores is thrown via
std::rethrow_exception().

The main benefit is than now any exception thrown via plain 'throw'
can be trasnported via boost::exception_ptr. Before this change it was
required that the throw site either used boost::enable_current_exception()
or boost::throw_exception() or threw an exception type inheriting from
boost::exception, which was impossible for third party software
that does not use Boost.Exception.

The detection of std::current_exception() is currently done via config macro
BOOST_NO_CXX11_NOEXCEPT, assuming that if 'noexcept' is supported then
std::exception_ptr is also supported. A better solution would require a new
dedicated macro in Boost.Config.

* Detect support for std::current_exception() via config macro BOOST_NO_CXX11_HDR_EXCEPTION

The temporary solution via BOOST_NO_CXX11_NOEXCEPT was an ugly hack
that is no longer necessary now that Boost.Config has
BOOST_NO_CXX11_HDR_EXCEPTION (pending merge to develop).

* Attempt to solve issue #10

In boost::current_exception(), after all supported types of exception have
been checked for, a boost::unknown_exception is stored in the
boost::exception_ptr as a last resort to signal that an exception was indeed
stored, but that the actual type and value of the exception was lost.

Now, in case C++11 std::current_exception() is supported, the
std::exception_ptr result of calling std::current_exception() is what
gets stored inside the boost::exception_ptr. Later, inside
boost::rethrow_exception, the std::exception_ptr is retrieved from
the boost::exception_ptr and whatever exception it stores is thrown via
std::rethrow_exception().

The main benefit is than now any exception thrown via plain 'throw'
can be trasnported via boost::exception_ptr. Before this change it was
required that the throw site either used boost::enable_current_exception()
or boost::throw_exception() or threw an exception type inheriting from
boost::exception, which was impossible for third party software
that does not use Boost.Exception.

The detection of std::current_exception() is currently done via config macro
BOOST_NO_CXX11_NOEXCEPT, assuming that if 'noexcept' is supported then
std::exception_ptr is also supported. A better solution would require a new
dedicated macro in Boost.Config.

* Detect support for std::current_exception() via config macro BOOST_NO_CXX11_HDR_EXCEPTION

The temporary solution via BOOST_NO_CXX11_NOEXCEPT was an ugly hack
that is no longer necessary now that Boost.Config has
BOOST_NO_CXX11_HDR_EXCEPTION (pending merge to develop).

* Document detection of C++11 std::current_exception through BOOST_NO_CXX11_HDR_EXCEPTION

* Added dist: trusty for Travis config, as recommended by pdimov in issue #10

* Stupid gcc 4.7 and 4.8 are confused initializing a reference with braces, using equal sign hoping it works (tested similar code on godbolt).

include/boost/exception/detail/exception_ptr.hpp

@@ -412,11 +412,14 @@ boost
                         {
                         return exception_detail::current_exception_std_exception(e);
                         }
+#ifdef BOOST_NO_CXX11_HDR_EXCEPTION
+                    // this case can be handled losslesly with std::current_exception() (see below)
                     catch(
                     std::exception & e )
                         {
                         return exception_detail::current_exception_unknown_std_exception(e);
                         }
+#endif
                     catch(
                     boost::exception & e )
                         {
@@ -425,7 +428,22 @@ boost
                     catch(
                     ... )
                         {
+#ifndef BOOST_NO_CXX11_HDR_EXCEPTION
+                        try
+                            {
+                            // wrap the std::exception_ptr in a clone-enabled Boost.Exception object
+                            exception_detail::clone_base const& base =
+                                boost::enable_current_exception(std::current_exception());
+                            return exception_ptr(shared_ptr<exception_detail::clone_base const>(base.clone()));
+                            }
+                        catch(
+                        ...)
+                            {
+                            return exception_detail::current_exception_unknown_exception();
+                            }
+#else
                         return exception_detail::current_exception_unknown_exception();
+#endif
                         }
                     }
                 }
@@ -461,7 +479,20 @@ boost
     rethrow_exception( exception_ptr const & p )
         {
         BOOST_ASSERT(p);
+#ifndef BOOST_NO_CXX11_HDR_EXCEPTION
+        try
+            {
+            p.ptr_->rethrow();
+            }
+        catch(
+        const std::exception_ptr& std_ep)
+            {
+            // if an std::exception_ptr was wrapped above then rethrow it
+            std::rethrow_exception(std_ep);
+            }
+#else
         p.ptr_->rethrow();
+#endif
         BOOST_ASSERT(0);
         #if defined(UNDER_CE)
             // some CE platforms don't define ::abort()