[M3devel] layout of objects?

[Dragiša Durić]

Yes, header too. And methods pointer is a structure from RT0 (methods and everything else). And data pointer program gets is address of first of fields.

On Mar 22, 2013, at 6:50 AM, Tony Hosking wrote:

> header
> methods pointer
> fields
> 
> If I recall correctly.
> 
> Sent from my iPad
> 
> On Mar 22, 2013, at 12:55 AM, Jay K <jay.krell at cornell.edu> wrote:
> 
>> layout of objects?
>> 
>> 
>> 
>> How are Modula-3 objects layed out?
>> i.e. "OBJECT"/"METHODS"/"OVERRIDES"
>> I skimmed m3front and it wasn't obvious.
>> 
>> 
>> 
>> A common way for C++ "objects" to be layed out,
>> in the face of no RTTI and only single inheritance,
>> and virtual functions, is that a pointer to a record
>> of function pointers is first in the record.
>> 
>> 
>> Like this:
>> 
>> 
>> class Type
>> {
>> virtual void F1();
>> virtual void F2();
>> int data1;
>> int data2;
>> };
>> 
>> 
>> ends up lik more this:
>> 
>> 
>> struct TypeFunctions
>> {
>>  void (*F1)(Type*);
>>  void (*F2)(Type*);
>> };
>> 
>> 
>> struct Type
>> {
>> TypeFunctions* Functions; /* always first,
>> or at least a fixed offset, and located independent
>> of the size of the data; could also be at "-1" or such */.
>> int data1;
>> int data2;
>> };
>> 
>> 
>> Type* x;
>> x->F1();
>> 
>> 
>> =>
>> x->Functions->F1(x);
>> 
>> 
>> Functions added in more derived types go at the end.
>> Ditto for data.
>> In the absence of multiple-inheritance and RTTI, it is simple and predictable.
>> (RTTI makes only small modifications.)
>> 
>> 
>> Looking through m3front, it wasn't at all obvious if it works this way.
>> 
>> 
>> I would like to declare something in C (or possibly C++, but not likely),
>> such that I might actually recognize the various low level operations
>> and "uncompile" it back to a typeful/typesafe form, like the above C++
>> to C transform.
>> 
>> 
>> 
>> I can't likely uncompile to C++ with virtual functions,
>> because the actual layout in C++ is not guaranteed.
>> 
>> 
>> 
>> Granted, I am being lazy.
>> I should/could compile some small samples.
>> But I might not get the entire story that way.
>> 
>> 
>> 
>> Thanks,
>> - Jay
>> 

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