Added the string_assign tool

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403876

dlib/string/string.h

@@ -336,6 +336,36 @@ namespace dlib
     template <typename T>
     inline const T string_cast (const wchar_t* str){ return string_cast<T>(std::wstring(str)); }
 
+// ----------------------------------------------------------------------------------------
+
+    class string_assign
+    {
+        class string_assign_helper
+        {
+        public:
+            string_assign_helper (const std::string& str_) : str(str_) {}
+
+            template <typename T>
+            operator T () const
+            {
+                return string_cast<T>(str);
+            }
+
+        private:
+
+            const std::string& str;
+        };
+
+    public:
+
+        string_assign_helper operator=(
+            const std::string& str
+        ) const
+        {
+            return string_assign_helper(str);
+        }
+    };
+
 // ----------------------------------------------------------------------------------------
 
     template <

dlib/string/string_abstract.h

@@ -38,6 +38,29 @@ namespace dlib
                 str into a T.  Also, string_cast_error::info == str
     !*/
 
+// ----------------------------------------------------------------------------------------
+
+    class string_assign
+    {
+        /*!
+            WHAT THIS OBJECT REPRESENTS
+                This is a simple tool which provides an alternative syntax for using
+                the string_cast() function.  It can be understood by considering
+                the following example:
+
+                    string_assign sa;
+                    int val;
+                    double dval;
+
+                    val  = sa = "1234";   // executes: val = string_cast<int>("1234");
+                    dval = sa = "3.141";  // executes: val = string_cast<double>("3.141");
+
+                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.
+        !*/
+    };
+
 // ----------------------------------------------------------------------------------------
 
     class cast_to_string_error : public error