Tuesday, March 13, 2012

type info stored inside instances - c#, c++, java

Java is simple and clean -- all non-primitive objects has a pointer to the runtime (not compile time[1]) class object, which lives in the permanent generation. This enables getClass().

C++ is similar. The RTTI info is often stored in the vtbl for each *virtual* class.  This means each java/c++ object needs only 4 bytes of real estate for vptr. (The type info is N bytes of real estate on the vtbl for the entire class.)

[1] Note the class object for the DECLARED type doesn't need to be stored in the instance. It's only used by the compiler and not needed at runtime. RTTI stands for "RunTime..."

----C# is less simple. See P54,104 [[c#precisely]]
- C# reference objects are probably same as java.
- C# struct instance has no "runtime type" and has no real estate carved out for it. A struct instance doesn't (need to) know it's own type.
** (C struct instance has no "runtime type" either, since C has no vtbl.)

--Boxing of struct type is trickier. Even though a struct instance has no pointer to the STRUCT type object, a boxed version, as all c# reference objects do, does hold a pointer to the CLASS object.

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