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, 6 insertions, 0 deletions

diff --git a/src/corelib/tools/qstring.cpp b/src/corelib/tools/qstring.cpp
index a5396bfb69..c73474684b 100644
--- a/src/corelib/tools/qstring.cpp
+++ b/src/corelib/tools/qstring.cpp
@@ -5782,6 +5782,12 @@ QString QString::trimmed_helper(QString &str)
     were a QChar &. If you assign to it, the assignment will apply to
     the character in the QString from which you got the reference.
 
+    \note Before Qt 5.14 it was possible to use this operator to access
+    a character at an out-of-bounds position in the string, and
+    then assign to such position, causing the string to be
+    automatically resized. This behavior is deprecated, and will be
+    changed in a future version of Qt.
+
     \sa at()
 */