Chapter 4. Commands to Edit Data

In this chapter, I will introduce DataCmd and its subclasses used to modify Data and its subclasses.

There are a lot of command objects introduced, and they can be classified as the following table.

Add	List::append, List::insert, Set::add, Map::add, Single::setValid
Remove	List::remove, List::clear, Set::remove, Set::clear, Map::remove, Map::clear, Single::setValid, Single::setNull
Move	List::move

Command for ownership and command for reference are provided for each of methods enumerated above.

	For Ownership	For Reference
Add”	Command create an Data instance and add to the container. Since it actually validates and invalidates the instance, it runs solution() as well.	Simply add an instance to the container.
Remove”	Command invalidates an instance and remove from the container. solution() is run as well.	Simply remove an existing reference.
Move”	Command simply move position of an instance in the same container. No Data::solution() is invoked.	Same with Ownership.

This is common traits of each classified command types.

You need to open the source code committed with message 004. Commands to edit Data before read the extra chapter.

Add-Ownership Commands

Open list_container_cmd.h.

Let’s see CreateAppendToList class template. Following the traits listed above, it actually creates a new Data instance and uses Data::solution() in undo() function, to solve the instance’s dependency. This is because undo() is opposite of creating new instance, which is deleting, which let the instance invalidated. Then it should not be referred by anywhere else, and it’s the reason why solution() is saved and called.

Attention that the class only creates the Data once, even though undo() is called after run() is called. This prevents the instance has different allocation position even though the command is once undone and redone.

Remove-Ownership Commands

RemoveFromList class template is an example of Remove-ownership command. Running it calls Data::solution()->run() so that every reference can be disconnected before it’s actually removed(invalidated). The solution is recovered(undone) when undo() is invoked, after the object is validated.

Move Commands

There’s no difference between Move-ownership and Move-reference, since in both cases no parent-child relationship breaking is occurred.

The class simply takes 2 indexes, moveThis and toHere, and call ListContainer::move(moveThis, toHere). Undoing swaps the parameters, into move(toHere, moveThis).

Add-Reference and Remove-Reference Commands

They are really simple just like Move commands. They take container and data instance, and add or remove from the container depends which one of run() or undo() is called.