Class instance memory overhead lower than 3 words?

[Nordlöw]

On Wednesday, 24 January 2018 at 21:47:26 UTC, H. S. Teoh wrote:
> On Wed, Jan 24, 2018 at 09:48:21PM +0000, Nordlöw via 
> Digitalmars-d-learn wrote:
>> Why is the memory overhead for a class instance as high as 3 
>> words (24 bytes on 64-bit systems? I find that annoyingly much 
>> for my knowledge database application.
> [...]
>
> There's been an attempt to get rid of the Monitor field, which 
> takes up one word, but it didn't seem to have gone anywhere the 
> last time it was discussed.  I believe originally it was always 
> initialized, but lately it seems to have been changed to be 
> lazily initialized, so at least you'll only incur the 
> computational cost if you actually use it. Nevertheless, the 
> field itself is still there.
>
>
> T

Can I use `*(cast(size_t*)&c.__monitor)` for storing my own stuff 
in a class instance `c` if I never use monitors? :)

BTW: the documentation on the concept of monitors is kind of 
sparse here

https://dlang.org/spec/class.html#class_properties

What is the `__monitor` property used for? Something with 
synchronized member functions?