Re:Baby's First Garbage Collector
by _delirium on Sunday December 08, 2013 @07:40PM
(#6872274)
Most languages don't give you any guarantees if you do that, though. In C, for example, pointer arithmetic on a void*, or on a pointer cast to an integer, is not guaranteed to produce sensible results. The standard only provides guarantees for pointer arithmetic if it's performed on a non-void pointer that points within an array, and the results remain within the bounds of the same array (arithmetic that produces a pointer pointing outside the array of the base pointer is undefined, with the exception that pointing to one element past the end of the array is defined).
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In ordinary C, without a garbage collector, I'd be baffled to find an implementation where one could not:
1) Cast a pointer to an integer of sufficient size.
2) Xor that integer with another value.
3) Xor the result with the same value.
4) Cast back to a pointer.
5) Dereference.
That is actually allowed in the standard; it specifically says that casting a pointer to an integral type of sufficient size is a reversible operation (though it I'm not sure if this is not well defined: (int i[2]; (int )((int)i)+sizeof(int)
Could you have a standards-conforming implementation where pointers were big-endian and other integral values little-endian? That would break your example code. I don't know of any architecture that would motivate that, of course...