Improved the string_assign utility.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403877

dlib/string/string.h

@@ -340,10 +340,17 @@ namespace dlib
 
     class string_assign
     {
+        template <
+            typename charT,
+            typename traits,
+            typename alloc
+            >
         class string_assign_helper
         {
         public:
-            string_assign_helper (const std::string& str_) : str(str_) {}
+            string_assign_helper (
+                const std::basic_string<charT,traits,alloc>& str_
+            ) : str(str_) {}
 
             template <typename T>
             operator T () const
@@ -353,19 +360,82 @@ namespace dlib
 
         private:
 
-            const std::string& str;
+            const std::basic_string<charT,traits,alloc>& str;
         };
 
+    // -------------
+
+        class char_assign_helper
+        {
         public:
+            char_assign_helper (
+                const char* str_
+            ) : str(str_) {}
+
+            template <typename T>
+            operator T () const
+            {
+                return string_cast<T>(str);
+            }
 
-        string_assign_helper operator=(
-            const std::string& str
+        private:
+
+            const char* str;
+        };
+
+    // -------------
+
+        class wchar_t_assign_helper
+        {
+        public:
+            wchar_t_assign_helper (
+                const wchar_t* str_
+            ) : str(str_) {}
+
+            template <typename T>
+            operator T () const
+            {
+                return string_cast<T>(str);
+            }
+
+        private:
+
+            const wchar_t* str;
+        };
+
+    // -------------
+
+    public:
+
+        template <
+            typename charT,
+            typename traits,
+            typename alloc
+            >
+        string_assign_helper<charT,traits,alloc> operator=(
+            const std::basic_string<charT,traits,alloc>& str
+        ) const
+        {
+            return string_assign_helper<charT,traits,alloc>(str);
+        }
+
+        char_assign_helper operator= (
+            const char* str
+        ) const 
+        {
+            return char_assign_helper(str);
+        }
+
+        wchar_t_assign_helper operator= (
+            const wchar_t* str
         ) const 
         {
-            return string_assign_helper(str);
+            return wchar_t_assign_helper(str);
         }
     };
 
+    const string_assign sa = string_assign();
+
 // ----------------------------------------------------------------------------------------
 
     template <

dlib/string/string_abstract.h

@@ -58,9 +58,15 @@ namespace dlib
                 After executing, val will be equal to 1234 and dval will be 3.141.
                 Note that you can use string_assign to assign to any type which you could
                 use with string_cast().  You aren't just limited to ints and doubles.
+
+                Additionally, note that there is a global instance of this object, dlib::sa. 
+                So you never have to create a string_assign object yourself.  Finally, this 
+                object is totally stateless and threadsafe.   
         !*/
     };
 
+    const string_assign sa = string_assign();
+
 // ----------------------------------------------------------------------------------------
 
     class cast_to_string_error : public error