Long live QStringView!

QStringView is a simple container for (const QChar*, int) and (const
char16_t*, size_t). It acts as a replacement interface type for const
QString and const QStringRef, and enables passing all kinds of
string-like types to functions otherwise expecting const QString& -
without the need to convert to QString first.

The use of this new class is guarded by a macro that enables three
levels of QStringView support:

 1. offer QStringView, overload some functions taking QString with
    QStringView

 2. like 1, but remove all overloads of functions taking QStringRef,
    leaving only the function taking QStringView.  Do this only where
    QStringRef overloads tradionally existed.

 3. like 2, but replace functions taking QString, too.

This is done in order to measure the impact of QStringView on code
size and execution speed, and to help guide the decision of which
level to choose for Qt 6.

This first patch adds QStringView with most of its planned
constructors, but not much more than iterators and isNull()/isEmpty().

Further patches will add support for QStringView to QStringBuilder,
add QStringView overloads of functions taking QString, and add the
complete API of const QString to QStringView.

[ChangeLog][QtCore][QStringView] New class, superseding const QString
and QStringRef as function parameters, accepting a wide variety of
UTF-16 string data sources, e.g. u"string", std::u16string{,_view},
and, on Windows, L"string", std::wstring{,_view} without converting to
QString first.

Change-Id: Iac273e46b2c61ec2c31b3dacebb29500599d6898
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 533 insertions, 0 deletions

diff --git a/src/corelib/tools/qstringview.cpp b/src/corelib/tools/qstringview.cpp
new file mode 100644
index 0000000000..8504b941b3
--- /dev/null
+++ b/src/corelib/tools/qstringview.cpp
@@ -0,0 +1,533 @@
+/****************************************************************************
+**
+** Copyright (C) 2017 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Marc Mutz <marc.mutz@kdab.com>
+** Contact: http://www.qt.io/licensing/
+**
+** This file is part of the QtCore module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include "qstringview.h"
+
+QT_BEGIN_NAMESPACE
+
+/*!
+    \class QStringView
+    \inmodule QtCore
+    \since 5.10
+    \brief The QStringView class provides a unified view on UTF-16 strings
+    \reentrant
+    \ingroup tools
+    \ingroup string-processing
+
+    QStringView provides a read-only subset of the QString API.
+
+    A string view explicitly stores a portion of a UTF-16 string it does
+    not own. It acts as an interface type to all kinds of UTF-16 string,
+    without the need to construct a QString first.
+
+    The UTF-16 string may be represented as an array (or an array-compatible
+    data-structure such as QString,
+    std::basic_string, etc.) of \c QChar, \c ushort, \c char16_t (on compilers that
+    support C++11 Unicode strings) or (on platforms, such as Windows,
+    where it is a 16-bit type) \c wchar_t.
+
+    QStringView is designed as an interface type; its main use-case is
+    as a function parameter type. When QStringViews are used as automatic
+    variables or data members, care must be taken to ensure that the referenced
+    string data (for example, owned by a QString) outlives the QStringView on all code paths,
+    lest the string view ends up referencing deleted data.
+
+    When used as an interface type, QStringView allows a single function to accept
+    a wide variety of UTF-16 string data sources. One function accepting QStringView
+    thus replaces three function overloads (taking QString, QStringRef, and
+    \c{(const QChar*, int)}), while at the same time enabling even more string data
+    sources to be passed to the function, such as \c{u"Hello World"}, a \c char16_t
+    string literal.
+
+    QStringViews should be passed by value, not by reference-to-const:
+    \code
+    void myfun1(QStringView sv);        // preferred
+    void myfun2(const QStringView &sv); // compiles and works, but slower
+    \endcode
+
+    If you want to give your users maximum freedom in what strings they can pass
+    to your function, accompany the QStringView overload with overloads for
+
+    \list
+        \li \e QChar: this overload can delegate to the QStringView version:
+            \code
+            void fun(QChar ch) { fun(QStringView(&ch, 1)); }
+            \endcode
+            even though, for technical reasons, QStringView cannot provide a
+            QChar constructor by itself.
+        \li \e QString: if you store an unmodified copy of the string and thus would
+            like to take advantage of QString's implicit sharing.
+        \li QLatin1String: if you can implement the function without converting the
+            QLatin1String to UTF-16 first; users expect a function overloaded on
+            QLatin1String to perform strictly less memory allocations than the
+            semantically equivalent call of the QStringView version, involving
+            construction of a QString from the QLatin1String.
+    \endlist
+
+    QStringView can also be used as the return value of a function. If you call a
+    function returning QStringView, take extra care to not keep the QStringView
+    around longer than the function promises to keep the referenced string data alive.
+    If in doubt, obtain a strong reference to the data by calling toString() to convert
+    the QStringView into a QString.
+
+    QStringView is a \e{Literal Type}.
+
+    \note We strongly discourage the use of QList<QStringView>,
+    because QList is a very inefficient container for QStringViews (it would heap-allocate
+    every element). Use QVector (or std::vector) to hold QStringViews instead.
+
+    \sa QString, QStringRef
+*/
+
+/*!
+    \typedef QStringView::value_type
+
+    Alias for \c{const QChar}. Provided for compatibility with the STL.
+*/
+
+/*!
+    \typedef QStringView::difference_type
+
+    Alias for \c{std::ptrdiff_t}. Provided for compatibility with the STL.
+*/
+
+/*!
+    \typedef QStringView::size_type
+
+    Alias for \c{std::ptrdiff_t}. Provided for compatibility with the STL.
+
+    Unlike other Qt classes, QStringView uses \c ptrdiff_t as its \c size_type, to allow
+    accepting data from \c{std::basic_string} without truncation. The Qt API functions,
+    for example length(), return \c int, while the STL-compatible functions, for example
+    size(), return \c size_type.
+*/
+
+/*!
+    \typedef QStringView::reference
+
+    Alias for \c{value_type &}. Provided for compatibility with the STL.
+
+    QStringView does not support mutable references, so this is the same
+    as const_reference.
+*/
+
+/*!
+    \typedef QStringView::const_reference
+
+    Alias for \c{value_type &}. Provided for compatibility with the STL.
+*/
+
+/*!
+    \typedef QStringView::pointer
+
+    Alias for \c{value_type *}. Provided for compatibility with the STL.
+
+    QStringView does not support mutable pointers, so this is the same
+    as const_pointer.
+*/
+
+/*!
+    \typedef QStringView::const_pointer
+
+    Alias for \c{value_type *}. Provided for compatibility with the STL.
+*/
+
+/*!
+    \typedef QStringView::iterator
+
+    This typedef provides an STL-style const iterator for QStringView.
+
+    QStringView does not support mutable iterators, so this is the same
+    as const_iterator.
+
+    \sa const_iterator, reverse_iterator
+*/
+
+/*!
+    \typedef QStringView::const_iterator
+
+    This typedef provides an STL-style const iterator for QStringView.
+
+    \sa iterator, const_reverse_iterator
+*/
+
+/*!
+    \typedef QStringView::reverse_iterator
+
+    This typedef provides an STL-style const reverse iterator for QStringView.
+
+    QStringView does not support mutable reverse iterators, so this is the
+    same as const_reverse_iterator.
+
+    \sa const_reverse_iterator, iterator
+*/
+
+/*!
+    \typedef QStringView::const_reverse_iterator
+
+    This typedef provides an STL-style const reverse iterator for QStringView.
+
+    \sa reverse_iterator, const_iterator
+*/
+
+/*!
+    \fn QStringView::QStringView()
+
+    Constructs a null string view.
+
+    \sa isNull()
+*/
+
+/*!
+    \fn QStringView::QStringView(std::nullptr_t)
+
+    Constructs a null string view.
+
+    \sa isNull()
+*/
+
+/*!
+    \fn QStringView::QStringView(QString::Null)
+    \internal
+
+    Constructs a null string view.
+
+    \sa isNull()
+*/
+
+/*!
+    \fn QStringView::QStringView(const Char *str, size_type len)
+
+    Constructs a string view on \a str with length \a len.
+
+    The range \c{[str,len)} must remain valid for the lifetime of this string view object.
+
+    Passing \c nullptr as \a str is safe if \a len is 0, too, and results in a null string view.
+
+    The behavior is undefined if \a len is negative or, when positive, if \a str is \c nullptr.
+
+    This constructor only participates in overload resolution if \c Char is a compatible
+    character type. The compatible character types are: \c QChar, \c ushort, \c char16_t and
+    (on platforms, such as Windows, where it is a 16-bit type) \c wchar_t.
+*/
+
+/*!
+    \fn QStringView::QStringView(const Char *str)
+
+    Constructs a string view on \a str. The length is determined
+    by scanning for the first \c{char16_t(0)}.
+
+    \a str must remain valid for the lifetime of this string view object.
+
+    Passing \c nullptr as \a str is safe and results in a null string view.
+
+    This constructor only participates in overload resolution if \c Char is a compatible
+    character type. The compatible character types are: \c QChar, \c ushort, \c char16_t and
+    (on platforms, such as Windows, where it is a 16-bit type) \c wchar_t.
+*/
+
+/*!
+    \fn QStringView::QStringView(const QString &str)
+
+    Constructs a string view on \a str.
+
+    \c{str.data()} must remain valid for the lifetime of this string view object.
+
+    The string view will be null if and only if \c{str.isNull()}.
+*/
+
+/*!
+    \fn QStringView::QStringView(const QStringRef &str)
+
+    Constructs a string view on \a str.
+
+    \c{str.data()} must remain valid for the lifetime of this string view object.
+
+    The string view will be null if and only if \c{str.isNull()}.
+*/
+
+/*!
+    \fn QStringView::QStringView(const StdBasicString &str)
+
+    Constructs a string view on \a str. The length is taken from \c{str.size()}.
+
+    \c{str.data()} must remain valid for the lifetime of this string view object.
+
+    This constructor only participates in overload resolution if \c StdBasicString is an
+    instantiation of \c std::basic_string with a compatible character type. The
+    compatible character types are: \c QChar, \c ushort, \c char16_t and
+    (on platforms, such as Windows, where it is a 16-bit type) \c wchar_t.
+
+    The string view will be empty if and only if \c{str.empty()}. It is unspecified
+    whether this constructor can result in a null string view (\c{str.data()} would
+    have to return \c nullptr for this).
+
+    \sa isNull(), isEmpty()
+*/
+
+/*!
+    \fn QString QStringView::toString() const
+
+    Returns a deep copy of this string view's data as a QString.
+
+    The return value will be the null QString if and only if this string view is null.
+
+    \warning QStringView can store strings with more than 2\sup{30} characters
+    while QString cannot. Calling this function on a string view for which size()
+    returns a value greater than \c{INT_MAX / 2} constitutes undefined behavior.
+*/
+
+/*!
+    \fn const QChar *QStringView::data() const
+
+    Returns a const pointer to the first character in the string.
+
+    \sa begin(), end()
+*/
+
+/*!
+    \fn QStringView::const_iterator QStringView::begin() const
+
+    Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the first character in
+    the string.
+
+    This function is provided for STL compatibility.
+
+    \sa end(), cbegin(), rbegin(), data()
+*/
+
+/*!
+    \fn QStringView::const_iterator QStringView::cbegin() const
+
+    Same as begin().
+
+    This function is provided for STL compatibility.
+
+    \sa cend(), begin(), crbegin(), data()
+*/
+
+/*!
+    \fn QStringView::const_iterator QStringView::end() const
+
+    Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the imaginary
+    character after the last character in the list.
+
+    This function is provided for STL compatibility.
+
+    \sa begin(), cend(), rend()
+*/
+
+/*! \fn QStringView::const_iterator QStringView::cend() const
+
+    Same as end().
+
+    This function is provided for STL compatibility.
+
+    \sa cbegin(), end(), crend()
+*/
+
+/*!
+    \fn QStringView::const_reverse_iterator QStringView::rbegin() const
+
+    Returns a const \l{STL-style iterators}{STL-style} reverse iterator pointing to the first
+    character in the string, in reverse order.
+
+    This function is provided for STL compatibility.
+
+    \sa rend(), crbegin(), begin()
+*/
+
+/*!
+    \fn QStringView::const_reverse_iterator QStringView::crbegin() const
+
+    Same as rbegin().
+
+    This function is provided for STL compatibility.
+
+    \sa crend(), rbegin(), cbegin()
+*/
+
+/*!
+    \fn QStringView::const_reverse_iterator QStringView::rend() const
+
+    Returns a \l{STL-style iterators}{STL-style} reverse iterator pointing to one past
+    the last character in the string, in reverse order.
+
+    This function is provided for STL compatibility.
+
+    \sa rbegin(), crend(), end()
+*/
+
+/*!
+    \fn QStringView::const_reverse_iterator QStringView::crend() const
+
+    Same as rend().
+
+    This function is provided for STL compatibility.
+
+    \sa crbegin(), rend(), cend()
+*/
+
+/*!
+    \fn bool QStringView::empty() const
+
+    Returns whether this string view is empty - that is, whether \c{size() == 0}.
+
+    This function is provided for STL compatibility.
+
+    \sa isEmpty(), isNull(), size(), length()
+*/
+
+/*!
+    \fn bool QStringView::isEmpty() const
+
+    Returns whether this string view is empty - that is, whether \c{size() == 0}.
+
+    This function is provided for compatibility with other Qt containers.
+
+    \sa empty(), isNull(), size(), length()
+*/
+
+/*!
+    \fn bool QStringView::isNull() const
+
+    Returns whether this string view is null - that is, whether \c{data() == nullptr}.
+
+    This functions is provided for compatibility with other Qt containers.
+
+    \sa empty(), isEmpty(), size(), length()
+*/
+
+/*!
+    \fn QStringView::size_type QStringView::size() const
+
+    Returns the size of this string view, in UTF-16 code points (that is,
+    surrogate pairs count as two for the purposes of this function, the same
+    as in QString and QStringRef).
+
+    \sa empty(), isEmpty(), isNull(), length()
+*/
+
+/*!
+    \fn int QStringView::length() const
+
+    Same as size(), except returns the result as an \c int.
+
+    This function is provided for compatibility with other Qt containers.
+
+    \warning QStringView can represent strings with more than 2\sup{31} characters.
+    Calling this function on a string view for which size() returns a value greater
+    than \c{INT_MAX} constitutes undefined behavior.
+
+    \sa empty(), isEmpty(), isNull(), size()
+*/
+
+/*!
+    \fn QChar QStringView::operator[](size_type n) const
+
+    Returns the character at position \a n in this string view.
+
+    The behavior is undefined if \a n is negative or not less than size().
+
+    \sa at(), front(), back()
+*/
+
+/*!
+    \fn QChar QStringView::at(size_type n) const
+
+    Returns the character at position \a n in this string view.
+
+    The behavior is undefined if \a n is negative or not less than size().
+
+    \sa operator[](), front(), back()
+*/
+
+/*!
+    \fn QChar QStringView::front() const
+
+    Returns the first character in the string. Same as first().
+
+    This function is provided for STL compatibility.
+
+    \warning Calling this function on an empty string view constitutes
+    undefined behavior.
+
+    \sa back(), first(), last()
+*/
+
+/*!
+    \fn QChar QStringView::back() const
+
+    Returns the last character in the string. Same as last().
+
+    This function is provided for STL compatibility.
+
+    \warning Calling this function on an empty string view constitutes
+    undefined behavior.
+
+    \sa front(), first(), last()
+*/
+
+/*!
+    \fn QChar QStringView::first() const
+
+    Returns the first character in the string. Same as front().
+
+    This function is provided for compatibility with other Qt containers.
+
+    \warning Calling this function on an empty string view constitutes
+    undefined behavior.
+
+    \sa front(), back(), last()
+*/
+
+/*!
+    \fn QChar QStringView::last() const
+
+    Returns the last character in the string. Same as back().
+
+    This function is provided for compatibility with other Qt containers.
+
+    \warning Calling this function on an empty string view constitutes
+    undefined behavior.
+
+    \sa back(), front(), first()
+*/
+
+QT_END_NAMESPACE