Add a macro to disable only some 8 bit/16 bit string conversions

This is an opt-in trade-off between type safety and user
code convenience.

QT_NO_CAST_FROM_ASCII is highly beneficial to avoid unintended
conversions from 8 bit data with potentially "unsuitable"
encodings to QString. However, it has the undesirable side-effect
to require user code to wrap character and string literals
in QLatin1Char(...) and QLatin1String(...) or use similar
construction, cluttering the code significantly.

This patch introduces a QT_RESTRICTED_CAST_FROM_ASCII macro
that works almost as QT_NO_CAST_FROM_ASCII, except that it
enables the QChar(char) constructor and adds an additional
QString(const char (&ch)[N]) constructor that matches
C++ string literals, but no arbitrary character pointers.
This avoids a significant share of the need to clutter the
user code by only a slight relaxation of the type-safety.

[ChangeLog][QtCore][QString] Added QT_RESTRICTED_CAST_FROM_ASCII
macro as less intrusive alternative to QT_NO_CAST_FROM_ASCII.

Change-Id: Iac72f1f90f81fbcae9bfb1fe68b0fec6ffb36c50
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

1 files changed, 34 insertions, 6 deletions

diff --git a/src/corelib/tools/qstring.cpp b/src/corelib/tools/qstring.cpp
index 2067605646..95e45c8d93 100644
--- a/src/corelib/tools/qstring.cpp
+++ b/src/corelib/tools/qstring.cpp
@@ -732,12 +732,30 @@ inline char qToLower(char ch)
 const QString::Null QString::null = { };
 
 /*!
+  \macro QT_RESTRICTED_CAST_FROM_ASCII
+  \relates QString
+
+  Defining this macro disables most automatic conversions from source
+  literals and 8-bit data to unicode QStrings, but allows the use of
+  the \c{QChar(char)} and \c{QString(const char (&ch)[N]} constructors,
+  and the \c{QString::operator=(const char (&ch)[N])} assignment operator
+  giving most of the type-safety benefits of QT_NO_CAST_FROM_ASCII
+  but does not require user code to wrap character and string literals
+  with QLatin1Char, QLatin1String or similar.
+
+  Using this macro together with source strings outside the 7-bit range,
+  non-literals, or literals with embedded NUL characters is undefined.
+
+  \sa QT_NO_CAST_FROM_ASCII, QT_NO_CAST_TO_ASCII
+*/
+
+/*!
   \macro QT_NO_CAST_FROM_ASCII
   \relates QString
 
   Disables automatic conversions from 8-bit strings (char *) to unicode QStrings
 
-  \sa QT_NO_CAST_TO_ASCII, QT_NO_CAST_FROM_BYTEARRAY
+  \sa QT_NO_CAST_TO_ASCII, QT_RESTRICTED_CAST_FROM_ASCII, QT_NO_CAST_FROM_BYTEARRAY
 */
 
 /*!
@@ -746,7 +764,7 @@ const QString::Null QString::null = { };
 
   disables automatic conversion from QString to 8-bit strings (char *)
 
-  \sa QT_NO_CAST_FROM_ASCII, QT_NO_CAST_FROM_BYTEARRAY
+  \sa QT_NO_CAST_FROM_ASCII, QT_RESTRICTED_CAST_FROM_ASCII, QT_NO_CAST_FROM_BYTEARRAY
 */
 
 /*!
@@ -760,7 +778,7 @@ const QString::Null QString::null = { };
   Note: This only works for compilers that support warnings for
   deprecated API.
 
-  \sa QT_NO_CAST_TO_ASCII, QT_NO_CAST_FROM_ASCII
+  \sa QT_NO_CAST_TO_ASCII, QT_NO_CAST_FROM_ASCII, QT_RESTRICTED_CAST_FROM_ASCII
 */
 
 /*!
@@ -994,6 +1012,9 @@ const QString::Null QString::null = { };
     \list
     \li \c QT_NO_CAST_FROM_ASCII disables automatic conversions from
        C string literals and pointers to Unicode.
+    \li \c QT_RESTRICTED_CAST_FROM_ASCII allows automatic conversions
+       from C characters and character arrays, but disables automatic
+       conversions from character pointers to Unicode.
     \li \c QT_NO_CAST_TO_ASCII disables automatic conversion from QString
        to C strings.
     \endlist
@@ -1311,6 +1332,12 @@ const QString::Null QString::null = { };
     can be useful if you want to ensure that all user-visible strings
     go through QObject::tr(), for example.
 
+    \note Defining QT_RESTRICTED_CAST_FROM_ASCII also disables
+    this constructor, but enables a \c{QString(const char (&ch)[N])}
+    constructor instead. Using non-literal input, or input with
+    embedded NUL characters, or non-7-bit characters is undefined
+    in this case.
+
     \sa fromLatin1(), fromLocal8Bit(), fromUtf8()
 */
 
@@ -1749,10 +1776,11 @@ QString &QString::operator=(const QString &other)
     Assigns \a str to this string. The const char pointer is converted
     to Unicode using the fromUtf8() function.
 
-    You can disable this operator by defining \c
-    QT_NO_CAST_FROM_ASCII when you compile your applications. This
-    can be useful if you want to ensure that all user-visible strings
+    You can disable this operator by defining \c QT_NO_CAST_FROM_ASCII
+    or \c QT_RESTRICTED_CAST_FROM_ASCII when you compile your applications.
+    This can be useful if you want to ensure that all user-visible strings
     go through QObject::tr(), for example.
+
 */
 
 /*! \fn QString &QString::operator=(char ch)