Using non-ODR violating placeholder implementation -- thanks to Joaquin Lopez Munoz of the Boost C++ Library community for the pointers -- instead of extern placeholders. Uses a similar approach used in Boost.None.

mikhail_beris · mikhail_beris · commit 9e2824e7ca94 · 2007-07-07T15:00:21.000Z
diff --git a/boost/network/message/transformers.hpp b/boost/network/message/transformers.hpp
@@ -14,11 +14,25 @@
 #include <boost/network/message/transformers/selectors.hpp>
 #include <boost/network/message/transformers/to_upper.hpp>
 
+#include <boost/type_traits.hpp>
+
 namespace boost { namespace network {
     namespace impl {
         template <class Algorithm, class Selector>
             struct get_real_algorithm {
-                typedef typename Algorithm::template type<Selector> type;
+				typedef typename boost::function_traits<
+					typename boost::remove_pointer<
+						Algorithm
+					>::type
+				>
+				::result_type::
+				template type<
+					typename boost::function_traits<
+						typename boost::remove_pointer<
+							Selector
+						>::type
+					>::result_type
+				> type;
             };
 
         template <class Algorithm, class Selector>
diff --git a/boost/network/message/transformers/selectors.hpp b/boost/network/message/transformers/selectors.hpp
@@ -9,12 +9,39 @@
 
 namespace boost { namespace network {
     namespace selectors {
-        struct source_selector { };
-        struct destination_selector { };
+        struct source_selector;
+        struct destination_selector;
     }; // namespace selectors
 
-    extern selectors::source_selector source_;
-    extern selectors::destination_selector destination_;
+	selectors::source_selector source_(selectors::source_selector);
+	selectors::destination_selector destination_(selectors::destination_selector);
+
+	namespace selectors {
+		struct source_selector {
+		private:
+			source_selector() {};
+			source_selector(source_selector const &) {};
+			friend source_selector boost::network::source_(source_selector);
+		};
+
+		struct destination_selector {
+		private:
+			destination_selector() {};
+			destination_selector(destination_selector const &) {};
+			friend destination_selector boost::network::destination_(destination_selector);
+		};
+	}; // namespace selectors
+
+	typedef selectors::source_selector (*source_selector_t)(selectors::source_selector);
+	typedef selectors::destination_selector (*destination_selector_t)(selectors::destination_selector);
+
+	inline selectors::source_selector source_(selectors::source_selector) {
+		return selectors::source_selector();
+	};
+
+	inline selectors::destination_selector destination_(selectors::destination_selector) {
+		return selectors::destination_selector();
+	};
 
 }; // namespace network
 
diff --git a/boost/network/message/transformers/to_upper.hpp b/boost/network/message/transformers/to_upper.hpp
@@ -49,12 +49,30 @@ namespace boost { namespace network {
 
     }; // namespace impl
 
-    struct to_upper_placeholder { 
-        template <class Selector>
-            struct type : public impl::to_upper_transformer<Selector> { };
-    } ;
+	namespace detail {
+		struct to_upper_placeholder_helper;
+	};
+
+	detail::to_upper_placeholder_helper to_upper_(detail::to_upper_placeholder_helper);
+
+	namespace detail {
+
+		struct to_upper_placeholder_helper {
+			template <class Selector>
+				struct type : public impl::to_upper_transformer<Selector> { };
+		private:
+			to_upper_placeholder_helper() {};
+			to_upper_placeholder_helper(to_upper_placeholder_helper const &) {};
+			friend to_upper_placeholder_helper boost::network::to_upper_(to_upper_placeholder_helper);
+		};
+
+	};
+
+	typedef detail::to_upper_placeholder_helper (*to_upper_placeholder)(detail::to_upper_placeholder_helper);
 
-    extern to_upper_placeholder to_upper_;
+	inline detail::to_upper_placeholder_helper to_upper_(detail::to_upper_placeholder_helper) {
+		return detail::to_upper_placeholder_helper();
+	};
 
 }; // namespace network
 
diff --git a/libs/network/test/Jamfile.v2 b/libs/network/test/Jamfile.v2
@@ -8,7 +8,7 @@ project network_test :
     requirements
         <include>../../../
         <source>/boost//unit_test_framework
-	<link>static
+#	<link>static
     ;
 
 unit-test message_test : message_test.cpp ;

Original file line number	Diff line number	Diff line change
`@@ -8,7 +8,7 @@ project network_test :`
`8`	`8`	`requirements`
`9`	`9`	`<include>../../../`
`10`	`10`	`<source>/boost//unit_test_framework`
`11`		`- <link>static`
	`11`	`+# <link>static`
`12`	`12`	`;`
`13`	`13`
`14`	`14`	`unit-test message_test : message_test.cpp ;`