c2d2757bcc
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> |
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