SFINAE-ize the server_base metafunction

deanberris · deanberris · commit ba96b4f161c6 · 2011-04-09T12:44:38.000+08:00
The server_base metafunction type serves as a means for choosing which implementation base
the server will be deriving from. The SFINAE technique is used to enable the appropriate
implementation of the type depending on the tag dispatch mechanism being utilized throughout
the implementation.
diff --git a/boost/network/protocol/http/server.hpp b/boost/network/protocol/http/server.hpp
@@ -16,19 +16,21 @@
 #include <boost/network/protocol/http/server/parameters.hpp>
 
 namespace boost { namespace network { namespace http {
-
+    
+    template <class Tag, class Handler, class Enable = void>
+    struct server_base {
+        typedef unsupported_tag<Tag> type;
+    };
+    
     template <class Tag, class Handler>
-    struct server_base :
-        mpl::if_<
-            is_async<Tag>
-            , async_server_base<Tag, Handler>
-            , typename mpl::if_<
-                is_sync<Tag>
-                , sync_server_base<Tag, Handler>
-                , unsupported_tag<Tag>
-            >::type
-        >
-    {};
+    struct server_base<Tag, Handler, typename enable_if<is_async<Tag> >::type> {
+        typedef async_server_base<Tag, Handler> type;
+    };
+    
+    template <class Tag, class Handler>
+    struct server_base<Tag, Handler, typename enable_if<is_sync<Tag> >::type> {
+        typedef sync_server_base<Tag, Handler> type;
+    };
 
     template <class Tag, class Handler>
     struct basic_server : server_base<Tag, Handler>::type
