Made the attribute_list of the xml parser a little more friendly by allowing

you to ask for attributes that don't exist and get a defined behavior (an
exception being thrown) rather than it being a contract violation.

dlib/xml_parser/xml_parser_kernel_1.h

@@ -49,7 +49,7 @@ namespace dlib
        
         public:
 
-            // These typedefs are here for backwards compatibily with previous versions of
+            // These typedefs are here for backwards compatibly with previous versions of
             // dlib.
             typedef xml_parser kernel_1a;
             typedef xml_parser kernel_1a_c;
@@ -103,7 +103,10 @@ namespace dlib
                     const std::string& key
                 ) const
                 {
-                    return list[key];
+                    if (is_in_list(key))
+                        return list[key];
+                    else
+                        throw xml_attribute_list_error("No XML attribute named " + key + " is present in tag.");
                 }
 
                 bool at_start (

dlib/xml_parser/xml_parser_kernel_interfaces.h

@@ -6,10 +6,24 @@
 #include <string>
 #include "../interfaces/enumerable.h"
 #include "../interfaces/map_pair.h"
+#include "../error.h"
 
 namespace dlib
 {
 
+// ----------------------------------------------------------------------------------------
+
+    class xml_attribute_list_error : public dlib::error 
+    {
+        /*!
+            WHAT THIS OBJECT REPRESENTS
+                This is an exception object thrown by attribute_list objects if you try to
+                access a non-existent attribute.
+        !*/
+    public:
+        xml_attribute_list_error(const std::string& msg) : dlib::error(msg){}
+    };
+
 // ----------------------------------------------------------------------------------------
 // ----------------------------------------------------------------------------------------
 // ----------------------------------------------------------------------------------------
@@ -43,11 +57,12 @@ namespace dlib
             const std::string& key
         ) const =0;
         /*!
-            requires
-                - is_in_list(key) == true
             ensures
-                - returns a const reference to the value associated with the 
-                  attribute named key.
+                if (is_in_list(key) == true) then
+                    - returns a const reference to the value associated with the attribute
+                      named key.
+                - else
+                    - throws xml_attribute_list_error
         !*/
 
     protected: