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unsafe: document the behavior of Sizeof on an interface

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unsafe.Sizeof() can return a different value than
reflect.TypeOf(x).Size() for a variable of an interface static type.
This change points out the difference in behavior, by emphasizing that
unsafe.Sizeof() only returns the size of the interface value itself,
rather than the size of the value stored in the interface.

Fixes #67465.

Change-Id: Ia6a809debb7970be171b0fc186209e5d161784e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/586275
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Nayef Ghattas 2024-05-16 23:04:29 +02:00 committed by Gopher Robot
parent 180ea45566
commit 1a349fc92b
1 changed files with 3 additions and 1 deletions
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4
src/unsafe/unsafe.go
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@ -187,7 +187,9 @@ type Pointer *ArbitraryType
// of a hypothetical variable v as if v was declared via var v = x.
// The size does not include any memory possibly referenced by x.
// For instance, if x is a slice, Sizeof returns the size of the slice
// descriptor, not the size of the memory referenced by the slice.
// descriptor, not the size of the memory referenced by the slice;
// if x is an interface, Sizeof returns the size of the interface value itself,
// not the size of the value stored in the interface.
// For a struct, the size includes any padding introduced by field alignment.
// The return value of Sizeof is a Go constant if the type of the argument x
// does not have variable size.