struct subtyping?

[Simen kjaeraas]

spir <denis.spir at gmail.com> wrote:

> I would like to learn about possible workarounds, or other alternative  
> approaches to such problems, if ever you now any.

Basically, you probably want to use alias this:

http://digitalmars.com/d/2.0/class.html#AliasThis

It lets you subtype structs by propagating member field/function access
to a member, if the struct or class does not implement the field or
function itself.

alias this is panacea - if a function expects a base struct, and returns
it after doing some alterations, you cannot cast the result to a subtype
(see below).


> Also, I would love to read about the rationale for _not_ having struct  
> type hierarchies (probably this would not be a big deal, since D has it  
> for classes (*)) Or maybe I have simply not correctly understood  
> available docs?

The main reason is the slicing problem. If the language lets a user store
a struct that is a subtype of another struct, and that adds fields, in a
location where space is only allocated for the base struct, the extra
fields are eaten by grues, and bugs appear.



-- 
Simen