Added the is_convertible template.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402731

dlib/algs.h

@@ -399,6 +399,29 @@ namespace dlib
         is_same_type();
     };
 
+// ----------------------------------------------------------------------------------------
+
+    /*!A is_convertible
+
+        This is a template that can be used to determine if one type is convertible 
+        into another type.
+
+        For example:
+            is_convertible<int,float>::value == true    // because ints are convertible to floats
+            is_convertible<int*,float>::value == false  // because int pointers are NOT convertible to floats
+    !*/
+
+    template <typename from, typename to>
+    struct is_convertible
+    {
+        struct yes_type { char a; };
+        struct no_type { yes_type a[2]; };
+        static const from& from_helper();
+        static yes_type test(to);
+        static no_type test(...);
+        const static bool value = sizeof(test(from_helper())) == sizeof(yes_type);
+    };
+
 // ----------------------------------------------------------------------------------------
 
     /*!A is_signed_type