Why is the constructor of B called?

[tcak via Digitalmars-d-learn]

[code]
import std.stdio;

class B {
	this() {
		writeln("B.constructor");
		foo();
	}

	void foo() {
		writeln("B.foo");
	}
}

class D : B {
	this() {
		writeln("D.constructor");
	}

	override void foo() {
		writeln("D.foo overrides B.foo");
	}
}

void main() {
	auto b = new D();
}
[/code]

Result:

B.constructor
D.foo overrides B.foo
D.constructor

----

There is no use of "super()" in the constructor of D, yet B's 
constructor is called when D is created. Why is that so?

I changed the constructor of D as follows:

[code]
	this() {
		super();
		writeln("D.constructor");
	}
[/code]

Results haven't changed at all. "super()" doesn't make any 
difference. What's going on?

I wouldn't expect B's constructor to be called at all unless 
"super" is used there.