diff options
Diffstat (limited to 'src/corelib/global/qglobal.cpp')
-rw-r--r-- | src/corelib/global/qglobal.cpp | 79 |
1 files changed, 76 insertions, 3 deletions
diff --git a/src/corelib/global/qglobal.cpp b/src/corelib/global/qglobal.cpp index c02528491e..1227445f1e 100644 --- a/src/corelib/global/qglobal.cpp +++ b/src/corelib/global/qglobal.cpp @@ -1,7 +1,7 @@ /**************************************************************************** ** ** Copyright (C) 2016 The Qt Company Ltd. -** Copyright (C) 2015 Intel Corporation +** Copyright (C) 2016 Intel Corporation. ** Contact: https://www.qt.io/licensing/ ** ** This file is part of the QtCore module of the Qt Toolkit. @@ -3540,7 +3540,11 @@ int qrand() \snippet code/src_corelib_global_qglobal.cpp 33 - \sa Q_FOREACH() + \note Since Qt 5.7, the use of this macro is discouraged. It will + be removed in a future version of Qt. Please use C++11 range-for, + possibly with qAsConst(), as needed. + + \sa qAsConst() */ /*! @@ -3552,7 +3556,76 @@ int qrand() This macro is available even when \c no_keywords is specified using the \c .pro file's \c CONFIG variable. - \sa foreach() + \note Since Qt 5.7, the use of this macro is discouraged. It will + be removed in a future version of Qt. Please use C++11 range-for, + possibly with qAsConst(), as needed. + + \sa qAsConst() +*/ + +/*! + \fn qAsConst(T &t) + \relates <QtGlobal> + \since 5.7 + + Returns \a t cast to \c{const T}. + + This function is a Qt implementation of C++17's std::as_const(), + a cast function like std::move(). But while std::move() turns + lvalues into rvalues, this function turns non-const lvalues into + const lvalues. Like std::as_const(), it doesn't work on rvalues, + because it cannot be efficiently implemented for rvalues without + leaving dangling references. + + Its main use in Qt is to prevent implicitly-shared Qt containers + from detaching: + \code + QString s = ...; + for (QChar ch : s) // detaches 's' (performs a deep-copy if 's' was shared) + process(ch); + for (QChar ch : qAsConst(s)) // ok, no detach attempt + process(ch); + \endcode + + Of course, in this case, you could (and probably should) have declared + \c s as \c const in the first place: + \code + const QString s = ...; + for (QChar ch : s) // ok, no detach attempt on const objects + process(ch); + \endcode + but often that is not easily possible. + + It is important to note that qAsConst() does not copy its argument, + it just performs a \c{const_cast<const T&>(t)}. This is also the reason + why it is designed to fail for rvalues: The returned reference would go + stale too soon. So while this works (but detaches the returned object): + \code + for (QChar ch : funcReturningQString()) + process(ch); // OK, the returned object is kept alive for the loop's duration + \endcode + + this would not: + \code + for (QChar ch : qAsConst(funcReturningQString())) + process(ch); // ERROR: ch is copied from deleted memory + \endcode + + To prevent this construct from compiling (and failing at runtime), qAsConst() has + a second, deleted, overload which binds to rvalues. +*/ + +/*! + \fn qAsConst(const T &&t) + \relates <QtGlobal> + \since 5.7 + \overload + + This overload is deleted to prevent a dangling reference in code like + \code + for (QChar ch : qAsConst(funcReturningQString())) + process(ch); // ERROR: ch is copied from deleted memory + \endcode */ /*! |