Why can't I inherit (extend) structs?

Jarrett Billingsley kb3ctd2 at yahoo.com
Wed Oct 11 13:05:58 PDT 2006

"rm" <roel.mathys at gmail.com> wrote in message 
news:egj6fb$1ih4$1 at digitaldaemon.com...

> why do you say correct order?

It matters when you're dealing with system libraries that expect structures 
to be laid out in a certain way, and it matters if you want to have type 
polymorphism (i.e. casting a pointer to a derived structure to a base 
structure type).  I.e.

template BaseMembers()
    int x;

template DerivedMembers()
    int y;

struct Base
    mixin BaseMembers;

struct Derived
    mixin DerivedMembers; // oops!  wrong order
    mixin BaseMembers;

void main()
    Derived d;
    d.x = 3;
    d.y = 4;

    Base* b = cast(Base*)&d;

    writefln(b.x); // prints "4", not "3"

The compiler can't complain about something like this because it doesn't 
know anything about deriving structs and keeping their members in the 
correct order in memory.  With the ability to derive structs, the problem 
just goes away. 

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