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sql: relax constraints on VirtualCursor layout

VirtualCursor::FromSqliteCursor required that VirtualCursor had a
standard layout, but in fact VirtualCursor shouldn't have a standard
layout, and the fact that it does with libc++ is a deviation from the
C++ standard. This change:

1. Relaxes the requirement that VirtualCursor has a standard layout, and
2. Relaxes the requirement that the sqlite_cursor_ field has to be at
   offset 0

by use of offsetof() and pointer subtraction. This change both improves
standards compliance and makes this code build with libstdc++.

Bug: 1380656
Change-Id: I9c47abd9197b187da0360ca5619ccf7dadab4f33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4292313
Reviewed-by: Austin Sullivan <asully@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1111925}
This commit is contained in:
Elly Fong-Jones
2023-03-02 00:15:11 +00:00
committed by Chromium LUCI CQ
parent 5d4a2653a4
commit 7d1394bd63

@ -63,12 +63,10 @@ class VirtualCursor {
// |sqlite_cursor| must have been returned by VirtualTable::SqliteCursor().
static inline VirtualCursor* FromSqliteCursor(
sqlite3_vtab_cursor* sqlite_cursor) {
static_assert(std::is_standard_layout<VirtualCursor>::value,
"needed for the reinterpret_cast below");
static_assert(offsetof(VirtualCursor, sqlite_cursor_) == 0,
"sqlite_cursor_ must be the first member of the class");
VirtualCursor* result = reinterpret_cast<VirtualCursor*>(sqlite_cursor);
DCHECK_EQ(sqlite_cursor, &result->sqlite_cursor_);
VirtualCursor* result = reinterpret_cast<VirtualCursor*>(
(reinterpret_cast<char*>(sqlite_cursor) -
offsetof(VirtualCursor, sqlite_cursor_)));
CHECK_EQ(sqlite_cursor, &result->sqlite_cursor_);
return result;
}