No struct extending?

[Stewart Gordon]

Steve Horne wrote:
> In C++, 'struct' is (almost) a synonym for 'class'. More a declaration
> of intent than a different thing. One useful side-effect of this is
> that you can declare structs as extensions of other structs. This can
> be useful even for plain-old-data. e.g. data structures with several
> node types, but some shared fields. Inheritance doesn't always imply
> virtual tables and stuff.

I can imagine that such a feature could be useful indeed.  However, the 
approach you propose has a caveat.

The problem is that the extending concepts currently in D - class 
inheritance, interface implementation and enum base types - satisfy the 
"is a" relationship.  Basically, given

     class Qwert : Yuiop

or

     enum Asdfg : int

you can keep any Qwert in a variable of type Yuiop, or any Asdfg in a 
variable of type int, without losing any information.  This won't be 
true of structs if we did what you're suggesting.

Of course, it would at least make sense to somebody coming from a C++ 
background.  But it will be necessary to know whether something's a 
struct or a class in order to be sure whether converting it to the base 
type would lead to loss of data.

<snip>
> Adding a union...
> 
>   union u_Variant
>   {
>     s_Common m_Common;
>     s_Branch m_Branch;
>     s_Leaf   m_Leaf;
>   }
> 
> just means you have to specify 'm_Branch' or 'm_Leaf' for the
> non-shared fields too.
<snip>

In a union?  Doesn't seem to make sense.

But an idea would be to have a means whereby a defined struct can be 
used as an anonymous struct.  It would also be handy if a struct 
instance could be both anonymous and named, if you know what I mean.

But you remind me of an idea I've had for a while, which is being able 
to derive a union from a union or struct.  This would be used to wrap a 
structure from an external API and give it methods or alternative views 
of the data.

Stewart.

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