Ensure left/right/mid behave in a compatible way

QString and QStringRef did bounds checking for left/right/mid, whereas
QStringView was asserting on out of bounds.

Relax the behavior for QStringView and do bounds checking on pos/n
as well. This removes a source of potentially hidden errors when porting
from QStringRef (or QString) to QStringView.

Unfortunately, one difference remains, where QByteArray::left/right()
behaves differently (and somewhat more sane) than QString and
QStringRef. We're keeping the difference here, as it has been around
for many years.

Mark left/right/mid as obsolete and to be replaced with the new
first/last/slice methods.

Change-Id: I18c203799ba78c928a4610a6038089f27696c22e
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

5 files changed, 99 insertions, 68 deletions

diff --git a/src/corelib/text/qbytearray.cpp b/src/corelib/text/qbytearray.cpp
index 008ce7298b..3f83e36d90 100644
--- a/src/corelib/text/qbytearray.cpp
+++ b/src/corelib/text/qbytearray.cpp
@@ -2914,13 +2914,17 @@ bool QByteArray::isLower() const
     Returns a byte array that contains the first \a len bytes of this byte
     array.
 
+    \obsolete Use first() instead in new code.
+
     The entire byte array is returned if \a len is greater than
     size().
 
+    Returns an empty QByteArray if \a len is smaller than 0.
+
     Example:
     \snippet code/src_corelib_text_qbytearray.cpp 27
 
-    \sa startsWith(), right(), mid(), chopped(), chop(), truncate()
+    \sa first(), last(), startsWith(), chopped(), chop(), truncate()
 */
 
 QByteArray QByteArray::left(int len)  const
@@ -2935,15 +2939,18 @@ QByteArray QByteArray::left(int len)  const
 /*!
     Returns a byte array that contains the last \a len bytes of this byte array.
 
+    \obsolete Use last() instead in new code.
+
     The entire byte array is returned if \a len is greater than
     size().
 
+    Returns an empty QByteArray if \a len is smaller than 0.
+
     Example:
     \snippet code/src_corelib_text_qbytearray.cpp 28
 
-    \sa endsWith(), left(), mid(), chopped(), chop(), truncate()
+    \sa endsWith(), last(), first(), slice(), chopped(), chop(), truncate()
 */
-
 QByteArray QByteArray::right(int len) const
 {
     if (len >= size())
@@ -2957,6 +2964,8 @@ QByteArray QByteArray::right(int len) const
     Returns a byte array containing \a len bytes from this byte array,
     starting at position \a pos.
 
+    \obsolete Use slice() instead in new code.
+
     If \a len is -1 (the default), or \a pos + \a len >= size(),
     returns a byte array containing all bytes starting at position \a
     pos until the end of the byte array.
@@ -2964,13 +2973,15 @@ QByteArray QByteArray::right(int len) const
     Example:
     \snippet code/src_corelib_text_qbytearray.cpp 29
 
-    \sa left(), right(), chopped(), chop(), truncate()
+    \sa first(), last(), slice(), chopped(), chop(), truncate()
 */
 
 QByteArray QByteArray::mid(int pos, int len) const
 {
+    qsizetype p = pos;
+    qsizetype l = len;
     using namespace QtPrivate;
-    switch (QContainerImplHelper::mid(size(), &pos, &len)) {
+    switch (QContainerImplHelper::mid(size(), &p, &l)) {
     case QContainerImplHelper::Null:
         return QByteArray();
     case QContainerImplHelper::Empty:
@@ -2982,7 +2993,7 @@ QByteArray QByteArray::mid(int pos, int len) const
     case QContainerImplHelper::Full:
         return *this;
     case QContainerImplHelper::Subset:
-        return QByteArray(d.data() + pos, len);
+        return QByteArray(d.data() + p, l);
     }
     Q_UNREACHABLE();
     return QByteArray();
diff --git a/src/corelib/text/qstring.cpp b/src/corelib/text/qstring.cpp
index 6707cc580e..5e3debcc34 100644
--- a/src/corelib/text/qstring.cpp
+++ b/src/corelib/text/qstring.cpp
@@ -4530,12 +4530,14 @@ QString QString::section(const QRegularExpression &re, int start, int end, Secti
     Returns a substring that contains the \a n leftmost characters
     of the string.
 
+    \obsolete Use first() instead in new code.
+
     The entire string is returned if \a n is greater than or equal
     to size(), or less than zero.
 
     \snippet qstring/main.cpp 31
 
-    \sa right(), mid(), startsWith(), chopped(), chop(), truncate()
+    \sa first(), last(), startsWith(), chopped(), chop(), truncate()
 */
 QString QString::left(int n)  const
 {
@@ -4548,12 +4550,14 @@ QString QString::left(int n)  const
     Returns a substring that contains the \a n rightmost characters
     of the string.
 
+    \obsolete Use last() instead in new code.
+
     The entire string is returned if \a n is greater than or equal
     to size(), or less than zero.
 
     \snippet qstring/main.cpp 48
 
-    \sa left(), mid(), endsWith(), chopped(), chop(), truncate()
+    \sa endsWith(), last(), first(), slice(), chopped(), chop(), truncate()
 */
 QString QString::right(int n) const
 {
@@ -4566,6 +4570,8 @@ QString QString::right(int n) const
     Returns a string that contains \a n characters of this string,
     starting at the specified \a position index.
 
+    \obsolete Use slice() instead in new code.
+
     Returns a null string if the \a position index exceeds the
     length of the string. If there are less than \a n characters
     available in the string starting at the given \a position, or if
@@ -4576,13 +4582,15 @@ QString QString::right(int n) const
 
     \snippet qstring/main.cpp 34
 
-    \sa left(), right(), chopped(), chop(), truncate()
+    \sa first(), last(), slice(), chopped(), chop(), truncate()
 */
 
 QString QString::mid(int position, int n) const
 {
+    qsizetype p = position;
+    qsizetype l = n;
     using namespace QtPrivate;
-    switch (QContainerImplHelper::mid(size(), &position, &n)) {
+    switch (QContainerImplHelper::mid(size(), &p, &l)) {
     case QContainerImplHelper::Null:
         return QString();
     case QContainerImplHelper::Empty:
@@ -4594,7 +4602,7 @@ QString QString::mid(int position, int n) const
     case QContainerImplHelper::Full:
         return *this;
     case QContainerImplHelper::Subset:
-        return QString(constData() + position, n);
+        return QString(constData() + p, l);
     }
     Q_UNREACHABLE();
     return QString();
@@ -9375,28 +9383,17 @@ QString &QString::setRawData(const QChar *unicode, int size)
     \sa crbegin(), rend(), cend()
 */
 
-/*! \fn QLatin1String QLatin1String::mid(int start) const
-    \since 5.8
-
-    Returns the substring starting at position \a start in this object,
-    and extending to the end of the string.
-
-    \note This function performs no error checking.
-    The behavior is undefined when \a start < 0 or \a start > size().
-
-    \sa left(), right(), chopped(), chop(), truncate()
-*/
-
 /*! \fn QLatin1String QLatin1String::mid(int start, int length) const
     \since 5.8
-    \overload
 
     Returns the substring of length \a length starting at position
     \a start in this object.
 
-    \note This function performs no error checking.
-    The behavior is undefined when \a start < 0, \a length < 0,
-    or \a start + \a length > size().
+    Returns a null string if the \a start index exceeds the
+    length of the string. If there are less than \a length characters
+    available in the string starting at \a start, or if
+    \a length is negative (default), the function returns all characters
+    that are available from \a start.
 
     \sa left(), right(), chopped(), chop(), truncate()
 */
@@ -9407,8 +9404,8 @@ QString &QString::setRawData(const QChar *unicode, int size)
     Returns the substring of length \a length starting at position
     0 in this object.
 
-    \note This function performs no error checking.
-    The behavior is undefined when \a length < 0 or \a length > size().
+    The entire string is returned if \a length is greater than or equal
+    to size(), or less than zero.
 
     \sa mid(), right(), chopped(), chop(), truncate()
 */
@@ -9419,8 +9416,8 @@ QString &QString::setRawData(const QChar *unicode, int size)
     Returns the substring of length \a length starting at position
     size() - \a length in this object.
 
-    \note This function performs no error checking.
-    The behavior is undefined when \a length < 0 or \a length > size().
+    The entire string is returned if \a length is greater than or equal
+    to size(), or less than zero.
 
     \sa mid(), left(), chopped(), chop(), truncate()
 */
@@ -10777,8 +10774,10 @@ QStringRef QString::rightRef(int n) const
 */
 QStringRef QStringRef::mid(int pos, int n) const
 {
+    qsizetype p = pos;
+    qsizetype l = n;
     using namespace QtPrivate;
-    switch (QContainerImplHelper::mid(m_size, &pos, &n)) {
+    switch (QContainerImplHelper::mid(m_size, &p, &l)) {
     case QContainerImplHelper::Null:
         return QStringRef();
     case QContainerImplHelper::Empty:
@@ -10786,7 +10785,7 @@ QStringRef QStringRef::mid(int pos, int n) const
     case QContainerImplHelper::Full:
         return *this;
     case QContainerImplHelper::Subset:
-        return QStringRef(m_string, pos + m_position, n);
+        return QStringRef(m_string, p + m_position, l);
     }
     Q_UNREACHABLE();
     return QStringRef();
diff --git a/src/corelib/text/qstring.h b/src/corelib/text/qstring.h
index c0b23c9733..efd57328bb 100644
--- a/src/corelib/text/qstring.h
+++ b/src/corelib/text/qstring.h
@@ -179,14 +179,26 @@ public:
     const_reverse_iterator rend() const noexcept { return const_reverse_iterator(begin()); }
     const_reverse_iterator crend() const noexcept { return const_reverse_iterator(begin()); }
 
-    Q_DECL_CONSTEXPR QLatin1String mid(int pos) const
-    { return Q_ASSERT(pos >= 0), Q_ASSERT(pos <= size()), QLatin1String(m_data + pos, m_size - pos); }
-    Q_DECL_CONSTEXPR QLatin1String mid(int pos, int n) const
-    { return Q_ASSERT(pos >= 0), Q_ASSERT(n >= 0), Q_ASSERT(pos + n <= size()), QLatin1String(m_data + pos, n); }
-    Q_DECL_CONSTEXPR QLatin1String left(int n) const
-    { return Q_ASSERT(n >= 0), Q_ASSERT(n <= size()), QLatin1String(m_data, n); }
-    Q_DECL_CONSTEXPR QLatin1String right(int n) const
-    { return Q_ASSERT(n >= 0), Q_ASSERT(n <= size()), QLatin1String(m_data + m_size - n, n); }
+    constexpr QLatin1String mid(int pos, int n = -1) const
+    {
+        qsizetype p = pos;
+        qsizetype l = n;
+        using namespace QtPrivate;
+        auto result = QContainerImplHelper::mid(size(), &p, &l);
+        return result == QContainerImplHelper::Null ? QLatin1String() : QLatin1String(m_data + p, l);
+    }
+    constexpr QLatin1String left(int n) const
+    {
+        if (size_t(n) >= size_t(size()))
+            n = size();
+        return QLatin1String(m_data, n);
+    }
+    constexpr QLatin1String right(int n) const
+    {
+        if (size_t(n) >= size_t(size()))
+            n = size();
+        return QLatin1String(m_data + m_size - n, n);
+    }
     Q_REQUIRED_RESULT Q_DECL_CONSTEXPR QLatin1String chopped(int n) const
     { return Q_ASSERT(n >= 0), Q_ASSERT(n <= size()), QLatin1String(m_data, m_size - n); }
 
diff --git a/src/corelib/text/qstringview.cpp b/src/corelib/text/qstringview.cpp
index b9dc9d2d41..42d18851c7 100644
--- a/src/corelib/text/qstringview.cpp
+++ b/src/corelib/text/qstringview.cpp
@@ -611,49 +611,48 @@ QT_BEGIN_NAMESPACE
 */
 
 /*!
-    \fn QStringView QStringView::mid(qsizetype start) const
-
-    Returns the substring starting at position \a start in this object,
-    and extending to the end of the string.
-
-    \note The behavior is undefined when \a start < 0 or \a start > size().
-
-    \sa left(), right(), chopped(), chop(), truncate()
-*/
-
-/*!
     \fn QStringView QStringView::mid(qsizetype start, qsizetype length) const
-    \overload
 
     Returns the substring of length \a length starting at position
     \a start in this object.
 
-    \note The behavior is undefined when \a start < 0, \a length < 0,
-    or \a start + \a length > size().
+    \obsolete Use slice() instead in new code.
 
-    \sa left(), right(), chopped(), chop(), truncate()
+    Returns an empty string view if \a start exceeds the
+    length of the string. If there are less than \a length characters
+    available in the string starting at \a start, or if
+    \a length is negative (default), the function returns all characters that
+    are available from \a start.
+
+    \sa first(), last(), slice(), chopped(), chop(), truncate()
 */
 
 /*!
     \fn QStringView QStringView::left(qsizetype length) const
 
+    \obsolete Use first() instead in new code.
+
     Returns the substring of length \a length starting at position
     0 in this object.
 
-    \note The behavior is undefined when \a length < 0 or \a length > size().
+    The entire string is returned if \a length is greater than or equal
+    to size(), or less than zero.
 
-    \sa mid(), right(), chopped(), chop(), truncate()
+    \sa first(), last(), slice(), startsWith(), chopped(), chop(), truncate()
 */
 
 /*!
     \fn QStringView QStringView::right(qsizetype length) const
 
+    \obsolete Use last() instead in new code.
+
     Returns the substring of length \a length starting at position
     size() - \a length in this object.
 
-    \note The behavior is undefined when \a length < 0 or \a length > size().
+    The entire string is returned if \a length is greater than or equal
+    to size(), or less than zero.
 
-    \sa mid(), left(), chopped(), chop(), truncate()
+    \sa first(), last(), slice(), endsWith(), chopped(), chop(), truncate()
 */
 
 /*!
diff --git a/src/corelib/text/qstringview.h b/src/corelib/text/qstringview.h
index 2b085c769d..6dbec21165 100644
--- a/src/corelib/text/qstringview.h
+++ b/src/corelib/text/qstringview.h
@@ -257,14 +257,24 @@ public:
 
     Q_REQUIRED_RESULT Q_DECL_CONSTEXPR QChar at(qsizetype n) const { return (*this)[n]; }
 
-    Q_REQUIRED_RESULT Q_DECL_CONSTEXPR QStringView mid(qsizetype pos) const
-    { return Q_ASSERT(pos >= 0), Q_ASSERT(pos <= size()), QStringView(m_data + pos, m_size - pos); }
-    Q_REQUIRED_RESULT Q_DECL_CONSTEXPR QStringView mid(qsizetype pos, qsizetype n) const
-    { return Q_ASSERT(pos >= 0), Q_ASSERT(n >= 0), Q_ASSERT(pos + n <= size()), QStringView(m_data + pos, n); }
-    Q_REQUIRED_RESULT Q_DECL_CONSTEXPR QStringView left(qsizetype n) const
-    { return Q_ASSERT(n >= 0), Q_ASSERT(n <= size()), QStringView(m_data, n); }
-    Q_REQUIRED_RESULT Q_DECL_CONSTEXPR QStringView right(qsizetype n) const
-    { return Q_ASSERT(n >= 0), Q_ASSERT(n <= size()), QStringView(m_data + m_size - n, n); }
+    Q_REQUIRED_RESULT constexpr QStringView mid(qsizetype pos, qsizetype n = -1) const
+    {
+        using namespace QtPrivate;
+        auto result = QContainerImplHelper::mid(size(), &pos, &n);
+        return result == QContainerImplHelper::Null ? QStringView() : QStringView(m_data + pos, n);
+    }
+    Q_REQUIRED_RESULT constexpr QStringView left(qsizetype n) const
+    {
+        if (size_t(n) >= size_t(size()))
+            n = size();
+        return QStringView(m_data, n);
+    }
+    Q_REQUIRED_RESULT constexpr QStringView right(qsizetype n) const
+    {
+        if (size_t(n) >= size_t(size()))
+            n = size();
+        return QStringView(m_data + m_size - n, n);
+    }
 
     Q_REQUIRED_RESULT constexpr QStringView first(qsizetype n) const
     { Q_ASSERT(n >= 0); Q_ASSERT(n <= size()); return QStringView(m_data, int(n)); }