values types for Java [message reformatted]

Wed Oct 22 19:15:00 GMT 2003

Jeff Sturm wrote:
> > Such types should be marked with some additional attributes in the
> > classfile.
>> Perhaps. For gcj's purposes it may be sufficient to generate compatible
> bytecode, and leverage struct optimizations when compiling from source.
>> A simple example:
>> struct S {
> int a,b;
> }
>> class C {
> int x,y;
> S s;
> }
>> might produce bytecode equivalent to:
>> class C {
> int x,y,s$a,s$b;
> }
>> I can imagine something similar for method parameters. Struct return
> values are a little trickier; they could be wrapped in some other object
> perhaps.
>
Maybe you could pass each struct field as out parameters (in practice as
reference parameter) automagically added by the compiler. Thus:
 S M(T x) { .... return s;...}
would be compiled as
 void M(T x, int* s$a, int* s$b) { ... s$a = s.a; s$b = s.b; return;....}
(I'm not sure whether this is possible in gcj, but I would try a solution
along these lines)
>> > Anyway, this restriction would be formulated in the language report as
> > "value types are final subclasses of Object". This restriction is often
> > mentioned and could make sense (Per proposed it). This would
automatically
> > remove the need for virtual dispatch.
>> But even Object has a vtable. I feel making value types a subclass of
> Object undermines the value of this proposal, having simple, lightweight
> stack-allocated structs.
>> You could disallow these types from having Object semantics, but that
> seems awkward to me.

This is why (in both proposals) structs cannot be casted to Object.
-Patrik