Composite pattern

入れ物と中身を同一視するパターン．

import std.stdio;
interface Element {
	uint countChilds();
	void append(Element c);
}
class Folder : Element {
	private Element[] elems;
	override uint countChilds() {
		uint sum;
		foreach (c; elems) {
			sum += 1 + c.countChilds();
		}
		return sum;
	}
	override void append(Element c) { elems ~= c; }
}
class File : Element {
	override uint countChilds() { return 0; }
	override void append(Element c) { throw new Exception("Append error."); }
}
void main() {
	Element w = new Folder; // 集合体
	Element x = new Folder; // 集合体
	Element y = new File;   // 要素
	Element z = new File;   // 要素
	w.append(x);
	x.append(y);
	x.append(z);
	writeNumChilds(w);
	writeNumChilds(x);
	writeNumChilds(y);
	writeNumChilds(z);
}
void writeNumChilds(Element c) { writeln(c, " ", c.countChilds()); }

a.Folder 3
a.Folder 2
a.File 0
a.File 0