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

[Ali Çehreli via Digitalmars-d-learn]

On 11/07/2016 08:40 AM, Heisenberg wrote:
> On Monday, 7 November 2016 at 16:33:30 UTC, Adam D. Ruppe wrote:
>> On Monday, 7 November 2016 at 16:22:17 UTC, Heisenberg wrote:
>>> Why? How can a delegate which returns nothing be used as an array
>>> which is going to be printed on the screen?
>>
>> You pass the string to the delegate, which does whatever with it
>> somewhere else.
>>
>> So you call: `passed_delegate("your string");` and it can forward it
>> to writeln or whatever.
>
> But how does it forward it if it's:
>
>> void delegate(const(char)[]) sink // ?
>
> It returns nothing, and just takes an array of characters as a
> parameter.. Just how does it print?..

Exactly how it happens requires explaining a long chain of function 
calls. Probably that's why the author did not elaborate further. ;) I've 
probably omitted some steps here but I think the following is close enough:

- writeln() that's in the sample code calls write() with the addition of 
'\n'.

- write() eventually calls formattedWrite() with a lockingTextWriter as 
the writer:

   https://github.com/dlang/phobos/blob/master/std/stdio.d#L1401

- formattedWrite() calls formatValue():

   https://github.com/dlang/phobos/blob/master/std/format.d#L2906

- formatValue() calls formatObject() and that's the one that makes a 
distinction between different overloads of toString():

   https://github.com/dlang/phobos/blob/master/std/format.d#L2805

- formatObject() calls put(), which finally uses the 'sink' parameter as 
an output range and fills in with characters.

Ali