Transitive Const in OO programming

[Regan Heath]

Alex Burton wrote:
> I am interested in what the best way to handle situations such as
> these in real D code.
> 
> Transitive const combined with proper OO (no statics etc) means that
> no state can be preserved in const methods, given that everything a
> class refers to should be through a member or argument.
> 
> Yes I know that I could make StateMachine not a state machine, but in
> reality there are state machines, and this one could be a mock one
> for testing.
> 
> It comes down to whether all things that a class may refer to are
> necessarily part of that class for the purposes of const.
> 
> In D 2.0 a call to a const method will not make any changes to
> anything that persists after the call returns unless it changes a
> mutable argument to that method. (ignoring statics) This would
> require state to be passed in to that method, which would require
> state to be passed into any const methods calling it, which breaks
> encapsulation.
> 
> The code below fails to compile because
> StateMachineWrapper::getResult calls a non const member of
> StateMachine
> 
> class StateMachine
> {
> 	int state;
> 	this()
> 	{
> 		state = 0;
> 	}
> 	void sendMessage()
> 	{
> 		state = 1;
> 	}
> 	const int getResult()
> 	{
> 		if (state == 1)
> 		{
> 			return 10;
> 			state = 0;
> 		}
> 		else
> 			throw new Exception("not in state to getResult");
> 	}
> };
> 
> class StateMachineWrapper
> {
> 	StateMachine mMachine;
> 	this()
> 	{
> 		mMachine = new StateMachine;
> 	}
> 	const int getResult()
> 	{
> 		mMachine.sendMessage();
> 		return mMachine.getResult();
> 	}
> };
> 
> 
> void main()
> {
> 	const(StateMachineWrapper) wrapper = new StateMachineWrapper;
> 	int x = wrapper.getResult();	
> }

StateMachine::getResult modifies state and cannot be 'const' either.  In 
this case I think you need a non-const StateMachine::reset to go back to 
state = 0;  It makes sense, especially if you want to call getResult 
several times for example.

Why call StateMachine::sendMessage inside stateMachineWrapper::getResult?

Why does StateMachineWrapper::getResult have to be const?

It seems making StateMachineWrapper::getResult non-const solves the problem.

Regan