peter green wrote: > surely this also means > > 1: there has to be rtti for every field in every class so the compiler can > follow the paths from the class
That's almost required, not only for classes but for all data types that contain references (pointers) to managed objects. It's not necessarily RTTI where such information resides, the compiler (Delphi, at least) already maintains such tables for fields with reference counted data types. These tables are used in the Initialize and Finalize procedures of every data structure. > 2: you can't safely cast between classes and untyped pointers or integers > (because those refs would be invisible to the gc) It's a matter of the compiler specific handling of managed objects. Remember that dynamic strings (AnsiString), arrays etc. and their casts already have to be handled by the compiler. You'll encounter no problems with pointers as long as the objects are kept alive by references which GC recognizes. Consider an AnsiString and an PChar into the string, then the pointer will become invalid when the string is moved or destroyed. Likewise a pointer to an object will become invalid when the object is destroyes. These are situations which you'll have to consider already, even without GC. > 3: the GC can't work with pointers to things for every class GC doesn't have to know about pointer types, it's sufficient to determine that an object at an address (pointer value) is referenced and consequently is alive. When such an object has fields with further references, then GC certainly must know about the data type of the object. That's why managed objects usually have a somewhat fixed standard layout, so that GC can find the class type and layout of the object from e.g. the VMT pointer. Alternatively managed objects can have an additional (hidden) reference to the layout information, stored before the begin of the object. Such a preamble also is used by classic memory managers, though with different contents, so that the MM can return the memory to the free memory pool when the object is free'd with e.g. FreeMem, Dispose or Destroy. At this level GC and traditional MM are not very different ;-) DoDi _______________________________________________ fpc-devel maillist - fpc-devel@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-devel
