Suppose you have a function that takes a parameter that you want to transform in some way, and you want to require that all future access to the parameter be done through the transformed version. One example is a wrapper class that does logging.\u00b9<\/p>\n
struct WidgetRefWrapper\r\n{\r\n WidgetRefWrapper(\r\n Widget& widget,\r\n Logger& logger) :\r\n m_widget(widget), m_logger(logger) {}\r\n \r\n void Toggle() try\r\n {\r\n m_logger.Log(\"Toggling the widget\");\r\n m_widget.Toggle();\r\n m_logger.Log(\"Toggled the widget\");\r\n } catch (...) {\r\n m_logger.Log(\"Exception while toggling the widget\");\r\n throw;\r\n }\r\n\r\nprivate:\r\n Widget& m_widget;\r\n Logger& m_logger;\r\n};\r\n\r\nvoid DoSomething(Widget& widget)\r\n{\r\n Logger& logger = GetCurrentLogger();\r\n WidgetWrapper wrapper(widget, logger);\r\n\r\n \/\/ Do not use the widget directly!\r\n \/\/ Always use the wrapper!\r\n\r\n if (needs_toggling) {\r\n wrapper.Toggle();\r\n }\r\n}\r\n<\/pre>\nYou want that “Do not use the widget directly!” comment to have some teeth. Can you “poison” the widget<\/code> parameter so it cannot be used any more?<\/p>\nOne idea is to split the method into two. The outer function does the preliminary wrapping, and the worker function does the bulk of the work.<\/p>\n
void DoSomething(Widget& widget)\r\n{\r\n Logger& logger = GetCurrentLogger();\r\n WidgetWrapper wrapper(widget, logger);\r\n\r\n DoSomethingWorker(wrapper, logger):\r\n}\r\n\r\nvoid DoSomethingWorker(\r\n WidgetRefWrapper& wrapper,\r\n Logger& logger)\r\n{\r\n if (needs_toggling) {\r\n wrapper.Toggle();\r\n }\r\n}\r\n<\/pre>\nSince the raw widget<\/code> is never passed to the worker function, there is no way to access it. If you type widget<\/code>, you get an undefined identifier.<\/p>\nOn the other hand, it also means that you have to pass all of the work you’ve done so far to the worker function. In this case, we pass the logger<\/code>.<\/p>\nAlso, somebody might see that you split the work into two functions and say, “What’s the point of a function that is called in only one place? I can just inline it!” and now you’re back where you started.<\/p>\n