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author | Giuseppe D'Angelo <giuseppe.dangelo@kdab.com> | 2019-05-19 14:35:52 +0200 |
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committer | Giuseppe D'Angelo <giuseppe.dangelo@kdab.com> | 2019-05-19 16:33:56 +0200 |
commit | c2d2757bccc68e1b981df059786c2e76f2969530 (patch) | |
tree | 8f1f9fd3c82500c0366e6ac1f1043db86cfc1101 /examples | |
parent | c9b7cc349a13b722ecd636ec4eb8e21f9f712add (diff) |
QString/QByteArray: detach immediately in operator[]
Unlike any other implicitly shared container, QString/QByteArray
have a "lazy detach" mechanism: their operator[] returns a
special object; assignment into that object will actually
detach.
In other words:
QString a("Hello");
QCharRef c = a[0]; // does not detach
c = 'J'; // detach happens here
This allows this behavior:
QString a("Hello");
QCharRef c = a[0];
QString b = a;
c = 'J'; // detach happens here
assert(a == "Jello");
assert(b == "Hello");
Note that this happens only with operator[] -- the mutating
iterator APIs instead detach immediately, making the above code
have visible side effects in b (at the end, b == "Jello").
The reasons for this special behavior seems to have been lost in
the dawn of time: this is something present all the way back
since Qt 2, maybe even Qt 1. Holding on to a "reference" while
taking copies of a container is documented [1] to be a bad idea,
so we shouldn't double check that the users don't do it.
This patch:
1) adds an immediate detach in operator[], just like all other
containers;
2) adds a warning in debug builds in case QByteRef/QCharRef is going
to cause a detach;
3) marks operator[] as [[nodiscard]] to warn users not using
Clazy about the (unintended) detach now happening in their code.
This paves the way for removal of QCharRef/QByteRef, likely in
Qt 7.
[1] https://doc.qt.io/qt-5/containers.html#implicit-sharing-iterator-problem
[ChangeLog][QtCore][QString] QString::operator[] detaches
immediately. Previously, the detach was delayed until a
modification was made to the string through the returned
QCharRef.
[ChangeLog][QtCore][QByteArray] QByteArray::operator[] detaches
immediately. Previously, the detach was delayed until a
modification was made to the byte array through the returned
QByteRef.
Change-Id: I9f77ae36759d80dc3202426a798f5b1e5fb2c2c5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Diffstat (limited to 'examples')
0 files changed, 0 insertions, 0 deletions