5.14 Implementing Undo and Redo with Functor

The GoF book (Gamma et al. 1995) recommends that undo be implemented as an additional Unexecute member function of the Command class. The problem is, you cannot ex press Unexecute in a generic way because the relation between doing something and undoing something is unpredictable. The Unexecute solution is appealing when you have a concrete Command class for each operation in your application, but the Functor-based approach favors using a single class that can be bound to different objects and member function calls.

Al Stevens's article in Dr. Dobb's Journal (Stevens 1998) is of great help in studying generic implementations of undo and redo. He has built a generic undo/redo library that you certainly should check out before rolling your own, with Functor or not.

It's all about data structures. The basic idea in undo and redo is that you have to keep an undo stack and a redo stack. As the user "does" something, such as typing a character, you push a different Functor on the undo stack. This means the Document::InsertChar member function is responsible for pushing the action that appropriately undoes the insertion (such as Document::DeleteChar). This puts the burden on the member function that does the stuff and frees the Functor from knowing how to undo itself.

Optionally, you may want to push on the redo stack a Functor consisting of the Document and a pointer to the Document::InsertChar member function, all bound to the actual character type. Some editors allow "retyping": After you type something and select Redo, that block of typing is repeated. The binding we built for Functor greatly helps with storing a call to Document::InsertChar for a given character, all encapsulated in a single Functor. Also, not only should the last character typed be repeated—that wouldn't be much of a feature—but the whole sequence of typing after the last nontyping action should be repeated. Chaining Functors enters into action here: As long as the user types, you append to the same Functor. This way you can treat later multiple keystrokes as a single action.

Document::InsertChar essentially pushes a Functor onto the undo stack. When the user selects Undo, that Functor will be executed and pushed onto the redo stack.

As you see, binding arguments and composition allows us to treat Functors in a very uniform way: No matter what kind of call there is, it's ultimately packed in a Functor. This considerably eases the task of implementing undo and redo.