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    "they are mixed with other data." It's not so much that they're "mixed" with other data. It's easy to use the C++ type system to see what's a pointer and what is not. The problem is that pointers frequently become other data. Hiding a pointer in an integer is an unfortunately common tool for many C-style APIs. Commented Jan 27, 2016 at 2:57
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    You don't even need undefined behaviour to screw up a garbage collector in c++. You could, for example, serialize a pointer to a file and read it in later. In the meanwhile, your process may not contain that pointer anywhere in its address space, so the garbage collector could collect that object, and then when you deserialize the pointer... Whoops. Commented Jan 27, 2016 at 4:36
  • @Bwmat "Even"? Writing pointers to a file like that seems a bit... far-fetched. Anyway, same serious problem plagues pointers to stack objects, they may be gone when you read the pointer back from file elsewhere in the code! Deserializing invalid pointer value is undefined behavior, don't do that. Commented Jan 27, 2016 at 6:03
  • If course, you would need to be careful if you are doing something like that. It was meant to be an example that, in general, a garbage collector can't work 'properly' in all cases in c++ (without changing the language) Commented Jan 27, 2016 at 6:06
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    @gnasher729: Ehm, no? Past-end-pointers are perfectly fine? Commented Jan 27, 2016 at 12:40