Can't understand the application of delegates in toString() functions

[Heisenberg via Digitalmars-d-learn]

On Monday, 7 November 2016 at 16:37:50 UTC, Mathias Lang wrote:
> On Monday, 7 November 2016 at 16:22:17 UTC, Heisenberg wrote:
>> [...]
>
> When you ask for a string (or more generally, an array), you 
> put a specific construct on the way the data should be. For 
> starters, the items have to be contiguous in memory. But most 
> of the time, they aren't, and you end up allocating memory to 
> put them contiguously.
>
> [...]

I see.. So we're basically adding stuff to it and then printing 
it all in one go. Still, the delegate in the example isn't 
defined, so how does it manage to output the result?