Add implicit conversion between compatible reference wrappers (refs #83)

include/boost/core/ref.hpp

@@ -10,6 +10,7 @@
 #include <boost/config.hpp>
 #include <boost/config/workaround.hpp>
 #include <boost/core/addressof.hpp>
+#include <boost/core/enable_if.hpp>
 
 //
 //  ref.hpp - ref/cref, useful helper functions
@@ -46,6 +47,26 @@ namespace boost
 
 #endif
 
+namespace detail
+{
+
+template< class Y, class T > struct ref_convertible
+{
+    typedef char (&yes) [1];
+    typedef char (&no)  [2];
+
+    static yes f( T* );
+    static no  f( ... );
+
+    enum _vt { value = sizeof( (f)( static_cast<Y*>(0) ) ) == sizeof(yes) };
+};
+
+struct ref_empty
+{
+};
+
+} // namespace detail
+
 // reference_wrapper
 
 /**
@@ -87,6 +108,14 @@ public:
 public:
 #endif
 
+    template<class Y> friend class reference_wrapper;
+
+    template<class Y> reference_wrapper( reference_wrapper<Y> r,
+        typename enable_if_c<boost::detail::ref_convertible<Y, T>::value,
+            boost::detail::ref_empty>::type = boost::detail::ref_empty() ): t_( r.t_ )
+    {
+    }
+
     /**
      @return The stored reference.
      @remark Does not throw.

test/Jamfile.v2

@@ -35,7 +35,10 @@ compile-fail ref_rv_fail4.cpp ;
 compile-fail ref_rv_fail5.cpp ;
 compile-fail ref_implicit_fail.cpp ;
 compile-fail ref_implicit_fail2.cpp ;
+compile-fail ref_implicit_fail3.cpp ;
+compile-fail ref_implicit_fail4.cpp ;
 run ref_cv_test.cpp ;
+run ref_conversion_test.cpp ;
 
 run eif_constructors.cpp ;
 run eif_dummy_arg_disambiguation.cpp ;

test/ref_conversion_test.cpp

@@ -0,0 +1,39 @@
+//  Implicit conversions between compatible reference wrappers
+//
+//  Copyright 2020 Peter Dimov
+//
+//  Distributed under the Boost Software License, Version 1.0.
+//  See accompanying file LICENSE_1_0.txt or copy at
+//  http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/ref.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+struct X
+{
+};
+
+struct Y: public X
+{
+};
+
+void f1( boost::reference_wrapper<X> r, Y * p )
+{
+    BOOST_TEST_EQ( r.get_pointer(), p );
+}
+
+void f2( boost::reference_wrapper<int const> r, int * p )
+{
+    BOOST_TEST_EQ( r.get_pointer(), p );
+}
+
+int main()
+{
+    Y y;
+    f1( boost::ref(y), &y );
+
+    int i = 0;
+    f2( boost::ref(i), &i );
+
+    return boost::report_errors();
+}

test/ref_implicit_fail3.cpp

@@ -0,0 +1,29 @@
+//
+// Incompatible reference_wrappers must not implicitly convert to each other
+//
+// Copyright 2020 Peter Dimov
+//
+// Distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt
+//
+
+#include <boost/ref.hpp>
+
+struct X
+{
+};
+
+struct Y
+{
+};
+
+void f( boost::reference_wrapper<X> )
+{
+}
+
+int main()
+{
+    Y y;
+    f( boost::ref(y) ); // should fail
+}

test/ref_implicit_fail4.cpp

@@ -0,0 +1,21 @@
+//
+// Incompatible reference_wrappers must not implicitly convert to each other
+//
+// Copyright 2020 Peter Dimov
+//
+// Distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt
+//
+
+#include <boost/ref.hpp>
+
+void f( boost::reference_wrapper< int const > )
+{
+}
+
+int main()
+{
+    long y = 0;
+    f( boost::ref(y) ); // should fail
+}