Improve "callable" detection in serializer stream interface

This version "sees" operator() in the base.

2 files changed, 10 insertions, 14 deletions

diff --git a/xml/serializer b/xml/serializer
index ee5779b..6251392 100644
--- a/xml/serializer
+++ b/xml/serializer
@@ -292,16 +292,11 @@ namespace xml
     std::size_t depth_;
   };
 
-  // Stream-like interface for serializer. If the passed argument
-  // is a function with the void f(serializer&) signature or is a
-  // function object with the void operator() (serializer&) const
-  // operator, then this function (object) is called with the passed
-  // serializer. Otherwise, the argument is passed to the serializer's
-  // characters() function.
+  // Stream-like interface for serializer. If the passed argument is
+  // callable with the serializer as its argument, then this function
+  // (object) is called with the passed serializer. Otherwise, the
+  // argument is passed to the serializer's characters() function.
   //
-  serializer&
-  operator<< (serializer&, void (*func) (serializer&));
-
   template <typename T>
   serializer&
   operator<< (serializer&, const T& value);
diff --git a/xml/serializer.ixx b/xml/serializer.ixx
index 465fede..59d329f 100644
--- a/xml/serializer.ixx
+++ b/xml/serializer.ixx
@@ -155,26 +155,27 @@ namespace xml
 
   namespace details
   {
-    // Detect whether T defines void operator(A) const.
+    // Detect whether T is callable with argument A.
     //
     template <typename T, typename A>
-    struct is_functor
+    struct is_callable
     {
       typedef char no[1];
       typedef char yes[2];
+      template <typename X> static X declval ();
 
-      template <typename X, X> struct check;
+      template <int> struct check;
 
       template <typename>
       static no& test (...);
 
       template <typename X>
-      static yes& test (check<void (X::*) (A) const, &X::operator ()>*);
+      static yes& test (check<sizeof (declval<X> () (declval<A> ()), 0)>*);
 
       static const bool value = sizeof (test<T> (0)) == sizeof (yes);
     };
 
-    template <typename T, bool = is_functor<T, serializer&>::value>
+    template <typename T, bool = is_callable<const T&, serializer&>::value>
     struct inserter;
 
     template <typename T>