path: root/src/corelib/tools/qstring.h
diff options
authorGiuseppe D'Angelo <>2019-05-19 14:35:52 +0200
committerGiuseppe D'Angelo <>2019-05-19 16:33:56 +0200
commitc2d2757bccc68e1b981df059786c2e76f2969530 (patch)
tree8f1f9fd3c82500c0366e6ac1f1043db86cfc1101 /src/corelib/tools/qstring.h
parentc9b7cc349a13b722ecd636ec4eb8e21f9f712add (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] [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 <>
Diffstat (limited to 'src/corelib/tools/qstring.h')
1 files changed, 10 insertions, 6 deletions
diff --git a/src/corelib/tools/qstring.h b/src/corelib/tools/qstring.h
index 5bc3a87832..0f7b015bef 100644
--- a/src/corelib/tools/qstring.h
+++ b/src/corelib/tools/qstring.h
@@ -271,9 +271,9 @@ public:
inline const QChar at(int i) const;
const QChar operator[](int i) const;
- QCharRef operator[](int i);
+ Q_REQUIRED_RESULT QCharRef operator[](int i);
const QChar operator[](uint i) const;
- QCharRef operator[](uint i);
+ Q_REQUIRED_RESULT QCharRef operator[](uint i);
Q_REQUIRED_RESULT inline QChar front() const { return at(0); }
Q_REQUIRED_RESULT inline QCharRef front();
@@ -1089,7 +1089,7 @@ public:
if (Q_LIKELY(i < s.d->size))
return s.d->data()[i];
#ifdef QT_DEBUG
- warn(EmittingClass::QCharRef);
+ warn(WarningType::OutOfRange, EmittingClass::QCharRef);
return 0;
@@ -1098,10 +1098,14 @@ public:
using namespace QtPrivate::DeprecatedRefClassBehavior;
if (Q_UNLIKELY(i >= s.d->size)) {
#ifdef QT_DEBUG
- warn(EmittingClass::QCharRef);
+ warn(WarningType::OutOfRange, EmittingClass::QCharRef);
s.resize(i + 1, QLatin1Char(' '));
} else {
+#ifdef QT_DEBUG
+ if (Q_UNLIKELY(!s.isDetached()))
+ warn(WarningType::DelayedDetach, EmittingClass::QCharRef);
s.d->data()[i] = c.unicode();
@@ -1215,9 +1219,9 @@ inline void QString::squeeze()
inline QString &QString::setUtf16(const ushort *autf16, int asize)
{ return setUnicode(reinterpret_cast<const QChar *>(autf16), asize); }
inline QCharRef QString::operator[](int i)
-{ Q_ASSERT(i >= 0); return QCharRef(*this, i); }
+{ Q_ASSERT(i >= 0); detach(); return QCharRef(*this, i); }
inline QCharRef QString::operator[](uint i)
-{ return QCharRef(*this, i); }
+{ detach(); return QCharRef(*this, i); }
inline QCharRef QString::front() { return operator[](0); }
inline QCharRef QString::back() { return operator[](size() - 1); }
inline QString::iterator QString::begin()