Multiple Inheritance, Mixins, and Constructors

[Ary Borenszweig]

Chris Williams escribió:
> Well so I had a case where I needed to do something like multiple inheritance. Here's an example solution:
> 
> interface IFoo {
>    void bar();
> }
> template TFoo() {
>    void bar() {
>       // code
>       // ...
>    }
> }
> 
> class Base {
>    // variables
> }
> 
> class Thing : Base, IFoo {
>    mixin TFoo;
> }
> 
> Well so that's all well and good. The problem comes if I have an object declared in TFoo. For instance:
> 
> template TFoo() {
>    SomeObject s_o;
> 
>    void bar() {
>       s_o.x = 10;
>       
>       // code
>       // ...
>    }
> }
> 
> It compiles fine, but when I go to run it, trying to set so.x gets an error because s_o hasn't been instantiated. If I try:
> 
> template TFoo() {
>    SomeObject s_o = new SomeObject();
> 
>    void bar() {
>       s_o.x = 10;
>       
>       // code
>       // ...
>    }
> }
> 
> Then I get a compile-time error that I can't do that. Now I'd love to define a constructor in TFoo that instantiated s_o, but I'm pretty sure that would conflict should anyone try to add a constructor to (for instance) class Thing.
> 
> My solution was to add an initFoo() definition in IFoo, and say that it needs to be called in the constructor of anyone using IFoo/TFoo, but that seems pretty wonky. Admittedly the whole interface/mixin thing is already a bit wonky, but it seems like there should be a solution to at least the object initialization problem.

You can do this:

template TFoo() {
	// Never use this variable directly :-P
	SomeObject _s_o;

	SomeObject s_o() {
		if (_s_o is null)
			_s_o = new SomeObject();
		return _s_o;
	}

	// And if you want to make it assignable too
	SomeObject s_o(SomeObject x) {
		_s_o = x;
	}
	
	void bar() {
		s_o.x = 10;
		// code
		// ...
	}
}