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, 22 insertions, 3 deletions

diff --git a/src/corelib/tools/qstring.h b/src/corelib/tools/qstring.h
index 1abb91eabe..1feb8e186c 100644
--- a/src/corelib/tools/qstring.h
+++ b/src/corelib/tools/qstring.h
@@ -1060,10 +1060,29 @@ public:
 
     // all this is not documented: We just say "like QChar" and let it be.
     inline operator QChar() const
-    { return i < s.d->size ? s.d->data()[i] : 0; }
+    {
+        using namespace QtPrivate::DeprecatedRefClassBehavior;
+        if (Q_LIKELY(i < s.d->size))
+            return s.d->data()[i];
+#ifdef QT_DEBUG
+        warn(EmittingClass::QCharRef);
+#endif
+        return 0;
+    }
     inline QCharRef &operator=(QChar c)
-    { if (i >= s.d->size) s.resize(i + 1, QLatin1Char(' ')); else s.detach();
-      s.d->data()[i] = c.unicode(); return *this; }
+    {
+        using namespace QtPrivate::DeprecatedRefClassBehavior;
+        if (Q_UNLIKELY(i >= s.d->size)) {
+#ifdef QT_DEBUG
+            warn(EmittingClass::QCharRef);
+#endif
+            s.resize(i + 1, QLatin1Char(' '));
+        } else {
+            s.detach();
+        }
+        s.d->data()[i] = c.unicode();
+        return *this;
+    }
 
     // An operator= for each QChar cast constructors
 #ifndef QT_NO_CAST_FROM_ASCII