Template to create a type and instantiate it

[cy via Digitalmars-d-learn]

On Friday, 5 February 2016 at 07:44:29 UTC, Rikki Cattermole 
wrote:
> That code is completely wrong anyway.

Well, obviously it's wrong. If I don't know correct code that 
will do what I want, then I can't tell you what I want using 
correct code.

> But you could do:
>
> alias Derp = TFoo;
> Derp obj;

I wasn't trying to make instances of TFoo. I was trying to make a 
type on the spot every time TFoo is used.

Sort of like std.functional.unaryFun!, which I found since 
looking around for information about it. It looks like the key to 
doing it is using "mixin" inside the template declaration itself. 
Also in realizing that "template" can be treated like its own 
source code for the purpose of mixins.

template Thing(alias code) {
	class Thing {
		int a, b;
		this() {
			mixin(code);
		}
		static Thing instance;
		static this() {
			instance = new Thing;
		}
	}
}
import std.stdio: writeln;

mixin Thing!q{
	writeln("a ",this.a," b ",this.b);
};

int main() {
	writeln("the instance exists... somewhere...");
	return 0;
}

You could also say alias Q = Thing!... if accessing Q.instance is 
important.