QCharRef/QByteRef: warn when triggering the resizing operator= behavior

The whole reason of QCharRef/QByteRef existence is to have a magic
operator=. This operator= delays the actual detach up to
the moment something is written into the Ref, thus avoiding
spurious detaches to the underlying string/byte array.

operator= has also an extra feature, it allows this code to
succeed:

  QString s("abc");
  s[10] = 'z';
  assert(s == "abc      z");

This last behavior is *extremely* surprising.

The problem with all of this is that this extra convenience is
outweighted by the massive pessimization in the codegen for
operator[]; by the maintenance burden (QChar APIs need to be
mirrored in QCharRef, etc.), and, for the automatic resize, by the
fact that it's an super-niche use case.

Cherry on top, std::basic_string does not do that, and no Qt or std
container does that. In other words: any other container-like class
exhibits UB for out of bounds access.

We can't just go and change behavior, though. This is something
coming all the way back from Qt 2 (maybe even Qt 1), which means we
can't deprecate it at short notice.

This patch simply adds a warning in debug builds in case the special
resizing behavior is triggered. While at it, removes some code
duplication in QByteRef.

[ChangeLog][QtCore][QString] The behavior of operator[] to allow
implicit resizing of the string has been deprecated, and will be
removed in a future version of Qt.

[ChangeLog][QtCore][QByteArray] The behavior of operator[] to allow
implicit detaching and resizing of the byte array has been
deprecated, and will be removed in a future version of Qt.

Change-Id: I3b5c5191167f12a606bcf6e513e6f304b220d675
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
 
 
 
 
 
 
 
 
 

1 files changed, 36 insertions, 5 deletions

diff --git a/src/corelib/tools/qbytearray.h b/src/corelib/tools/qbytearray.h
index d21cb9b363..5c7229e40d 100644
--- a/src/corelib/tools/qbytearray.h
+++ b/src/corelib/tools/qbytearray.h
@@ -528,6 +528,17 @@ inline void QByteArray::squeeze()
     }
 }
 
+namespace QtPrivate {
+namespace DeprecatedRefClassBehavior {
+    enum class EmittingClass {
+        QByteRef,
+        QCharRef,
+    };
+
+    Q_CORE_EXPORT Q_DECL_COLD_FUNCTION void warn(EmittingClass c);
+} // namespace DeprecatedAssignmentOperatorBehavior
+} // namespace QtPrivate
+
 class
 #if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
 Q_CORE_EXPORT
@@ -540,13 +551,33 @@ QByteRef {
     friend class QByteArray;
 public:
     inline operator char() const
-        { return i < a.d->size ? a.d->data()[i] : char(0); }
+    {
+        using namespace QtPrivate::DeprecatedRefClassBehavior;
+        if (Q_LIKELY(i < a.d->size))
+            return a.d->data()[i];
+#ifdef QT_DEBUG
+        warn(EmittingClass::QByteRef);
+#endif
+        return char(0);
+    }
     inline QByteRef &operator=(char c)
-        { if (i >= a.d->size) a.expand(i); else a.detach();
-          a.d->data()[i] = c;  return *this; }
+    {
+        using namespace QtPrivate::DeprecatedRefClassBehavior;
+        if (Q_UNLIKELY(i >= a.d->size)) {
+#ifdef QT_DEBUG
+            warn(EmittingClass::QByteRef);
+#endif
+            a.expand(i);
+        } else {
+            a.detach();
+        }
+        a.d->data()[i] = c;
+        return *this;
+    }
     inline QByteRef &operator=(const QByteRef &c)
-        { if (i >= a.d->size) a.expand(i); else a.detach();
-          a.d->data()[i] = c.a.d->data()[c.i];  return *this; }
+    {
+        return operator=(char(c));
+    }
     inline bool operator==(char c) const
     { return a.d->data()[i] == c; }
     inline bool operator!=(char c) const