Added a macro to assert on a type being a POD or a standard layout type.

This way I can change just this one macro to use the better C++0x
method of doing this check once C++0x becomes available.

dlib/algs.h

@@ -931,6 +931,25 @@ namespace dlib
         void* const data; 
     };
 
+// ----------------------------------------------------------------------------------------
+
+    /*A DLIB_ASSERT_HAS_STANDARD_LAYOUT 
+    
+        This macro is meant to cause a compiler error if a type doesn't have a simple
+        memory layout (like a C struct). In particular, types with simple layouts are
+        ones which can be copied via memcpy().
+        
+        
+        This was called a POD type in C++03 and in C++0x we are looking to check if 
+        it is a "standard layout type".  Once we can use C++0x we can change this macro 
+        to something that uses the std::is_standard_layout type_traits class.  
+        See: http://www2.research.att.com/~bs/C++0xFAQ.html#PODs
+    */
+    // Use the fact that in C++03 you can't put non-PODs into a union.
+#define DLIB_ASSERT_HAS_STANDARD_LAYOUT(type)   \
+    union  BOOST_JOIN(DAHSL_,__LINE__) { type TYPE_NOT_STANDARD_LAYOUT; };  \
+    typedef char BOOST_JOIN(DAHSL2_,__LINE__)[sizeof(BOOST_JOIN(DAHSL_,__LINE__))]; 
+
 // ----------------------------------------------------------------------------------------
 
 }