Re: Base class with member parameterized on type of extending class

[Jacob Carlborg via Digitalmars-d-learn]

On 2014-10-19 13:19, rcor wrote:

I'm trying to make a game, and would like to set up the following
hierarchy:
At any time, the game is in one Scene. Each scene has a state machine
that manages States of type T, where T is the type of the scene (e.g.
Overworld, Menu).

abstract class State!T {
   void update(T scene, float time, InputManager input);
   ...
}

class StateMachine!T { //manages states of type State!T }

abstract class Scene {
   alias T = // type of class extending Scene
   // methods pushState, popState, currentState access _stateMachine
   private StateMachine!T _stateMachine;
}

class MainMenu : Scene {
    // I want _stateMachine of type StateMachine!MainMenu
}

class Overworld : Scene {
   // I want _stateMachine of type StateMachine!Overworld
}

class MoveToLocation : State!Overworld {
   override void update(Overworld world, float time, InputManager input) {
     // access properties of Overworld here
   }
}

Within the Scene class, I've tried alias T = typeof(this), but that
appears to be resolved within Scene. This means that any Scene, such as
Overworld, have a state machine of type StateMachine!Scene rather than
StateMachine!Overworld. Since States are particular to a certain scene
and are designed to manipulate properties specific to that type of
scene, this would involve a lot of casting if States are not
parameterized. It feels like I need something like a class version of
the (this T) syntax used in templates.

This all smells a bit off though, so I wouldn't be surprised if the
answer is that I'm approaching this all wrong, but right now I'm not
seeing it.

You can always make Scene a template class:

abstract class Scene (T)
{
    private StateMachine!T _stateMachine;
}

class MainMenu : Scene!(MainMenu) {}

But I'm guessing you like to avoid that if possible.

--
/Jacob Carlborg