[Gc] Correct, portable array cleanup?

Neil Toronto ntoronto at cs.byu.edu
Tue Apr 19 03:04:24 PDT 2005


Okay, so I'm new and spamming the list already, but I think I may be 
onto something. From gc_cpp.h:

"Implementing clean-up of arrays correctly, portably, and in a way that 
preserves the correct exception semantics requires a language extension, 
e.g. the 'gc' keyword."

I've made a stab, and I don't know enough about exception semantics to 
say this is correct, but I think it is. (What ARE correct exception 
semantics during garbage collection?)

I've put the first draft online here:

http://axon.cs.byu.edu/~neil/array.h


Usage:

array<A> a(10); // allocates an array of 10 elements, handle is 'a'
array<B> b(5, "Ha ha"); // allocates an array of 5 elements, handle 'b', 
calls B::B(const char*) for each

for (int i = 0; i < a.size(); i++)
{
    a[i] = ...
    ... = b[i];
}


Advantages over naked pointers:

- Objects in the array have destructors called when the array is GCd.

- Can't point at an array of descendants with a base pointer. For example:

class A;
class B : public A;

A* p = new B[20]; // compiles fine - icky, bad and evil
array<A> p = array<B>(20); // illegal - can't convert from array<B> to 
array<A>

- Bounds-checked access and NULL-pointer check when NDEBUG isn't defined.

- Exception-safe allocation (can safely use "array<T_>(...)" as a 
function parameter - see http://www.gotw.ca/gotw/056.htm - 
"Exception-Safe Function Calls").

- Takes as much space as a naked pointer (sizeof(array<T_>) == sizeof(T_*)).


Disadvantages:

- Extra level of indirection - access is slower (working on this with a 
placement-new solution - hopefully I can make it portable)

- Contained object constructors are limited to 20 arguments (can be 
expanded to arbitrary numbers using recursion a la Boost)


Since I haven't played with the collector much, comments are very welcome.

Neil




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