What is a sink delegate?

[Joel via Digitalmars-d-learn]

On Tuesday, 30 September 2014 at 17:27:09 UTC, Adam D. Ruppe 
wrote:
> On Tuesday, 30 September 2014 at 17:22:44 UTC, Gary Willoughby 
> wrote:
>> What is a sink delegate?
>
> Instead of
>
> string toString() { return "foo"; }
>
> for example, you would use:
>
> void toString(void delegate(string) sink) { sink("foo"); }
>
> The sink argument there is then free to view and discard the 
> data or to make a private copy using whatever scheme it desires.


How do you use that toString? Maybe an example? Below is my 
failed effort.

import std.stdio;

struct Try {
	string name;
	long age;
	
	void toString(void delegate(string) sink) {
		sink("foo");
	}
}

void main() {
	Try t = Try("Joel", 35);
	writeln(t);
}