[M3devel] layout of objects?

[Tony Hosking]

Not quite.  I just checked RTAllocator.m3 and it is pointer to methods at offset 0, followed by fields.
Heap header is at -ADRSIZE(Header).
The type information in RT0 is used to initialize the object instances.

On Mar 22, 2013, at 1:34 AM, Dragiša Durić <dragisha at m3w.org> wrote:

> It's not m3front, it's RT0.
> 
> First data field is at offset 0, and pointer to type information is at -BYTESIZE(POINTER). Type information records are specified in RT0.
> 
> On Mar 22, 2013, at 5:56 AM, Jay K 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
>> 
> 



Antony Hosking | Associate Professor | Computer Science | Purdue University
305 N. University Street | West Lafayette | IN 47907 | USA
Mobile +1 765 427 5484





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