I get how. I don't get why. (more const stuff)

[Gregor Richards]

Disclaimer: Non D2 user :)

'const' is a compiler-defined contract to say that you won't change 
anything you don't own with respect to some variable. If that variable 
is a struct passed as an argument (and therefore passed by value), you 
DO own its members, but you do NOT own anything that its members point 
to. Restricting you from changing the content of a local variable is 
pointless, but const in this context would prevent you from changing 
anything it points to.

To put it a different way: const never prevents you from changing local 
data, it only prevents you from writing through pointers and references.

Redisclaimer: Any of this can be wrong, non D2 user :)

  - Gregor Richards