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Heap Allocation: Make wording of heap start prefix clearer (#854)

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Hofer Julian
2020-09-11 10:56:53 +02:00
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commit 685b8a94ac
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blog/content/second-edition/posts/10-heap-allocation/index.md
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@@ -625,7 +625,7 @@ current reference count is 2
reference count is 1 now reference count is 1 now
](qemu-alloc-showcase.png) ](qemu-alloc-showcase.png)
As expected, we see that the `Box` and `Vec` values live on the heap, as indicated by the pointer starting with `0x_4444_4444`. The reference counted value also behaves as expected, with the reference count being 2 after the `clone` call, and 1 again after one of the instances was dropped. As expected, we see that the `Box` and `Vec` values live on the heap, as indicated by the pointer starting with the `0x_4444_4444_*` prefix. The reference counted value also behaves as expected, with the reference count being 2 after the `clone` call, and 1 again after one of the instances was dropped.
The reason that the vector starts at offset `0x800` is not that the boxed value is `0x800` bytes large, but the [reallocations] that occur when the vector needs to increase its capacity. For example, when the vector's capacity is 32 and we try to add the next element, the vector allocates a new backing array with capacity 64 behind the scenes and copies all elements over. Then it frees the old allocation. The reason that the vector starts at offset `0x800` is not that the boxed value is `0x800` bytes large, but the [reallocations] that occur when the vector needs to increase its capacity. For example, when the vector's capacity is 32 and we try to add the next element, the vector allocates a new backing array with capacity 64 behind the scenes and copies all elements over. Then it frees the old allocation.