QString: add STL-style assign() [1/4]: non-(it,it) overloads

Implemented assign() methods for QString to align with the
criteria of std::basic_string, addressing the previously missing
functionality. This is a subset of the overloads provided by the
standard.

Reference:
https://en.cppreference.com/w/cpp/string/basic_string/assign

The assign(it, it) overload is a bit more complicated and will be
added in follow-up patches.

[ChangeLog][QtCore][QString] Added assign().

Task-number: QTBUG-106198
Change-Id: Ia1481d184865f46db872cf94c266fef83b962351
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

2 files changed, 56 insertions, 0 deletions

diff --git a/src/corelib/text/qstring.cpp b/src/corelib/text/qstring.cpp
index b6b6f3c8a1..de1409960a 100644
--- a/src/corelib/text/qstring.cpp
+++ b/src/corelib/text/qstring.cpp
@@ -3310,6 +3310,52 @@ QString &QString::append(QChar ch)
 */
 
 /*!
+    \fn QString &QString::assign(QAnyStringView v)
+    \since 6.6
+
+    Replaces the contents of this string with a copy of \a v and returns a
+    reference to this string.
+
+    The size of this string will be equal to the size of \a v, converted to
+    UTF-16 as if by \c{v.toString()}. Unlike QAnyStringView::toString(), however,
+    this function only allocates memory if the estimated size exceeds the capacity
+    of this string or this string is shared.
+
+    \sa QAnyStringView::toString()
+*/
+
+/*!
+    \fn QString &QString::assign(qsizetype n, QChar c)
+    \since 6.6
+
+    Replaces the contents of this string with \a n copies of \a c and
+    returns a reference to this string.
+
+    The size of this string will be equal to \a n, which has to be non-negative.
+
+    This function will only allocate memory if \a n exceeds the capacity of this
+    string or this string is shared.
+
+    \sa fill()
+*/
+
+QString &QString::assign(QAnyStringView s)
+{
+    if (s.size() <= capacity() && isDetached()) {
+        const auto offset = d.freeSpaceAtBegin();
+        if (offset)
+            d.setBegin(d.begin() - offset);
+        resize(0);
+        s.visit([this](auto input) {
+            this->append(input);
+        });
+    } else {
+        *this = s.toString();
+    }
+    return *this;
+}
+
+/*!
   \fn QString &QString::remove(qsizetype position, qsizetype n)
 
   Removes \a n characters from the string, starting at the given \a
diff --git a/src/corelib/text/qstring.h b/src/corelib/text/qstring.h
index 62eb64d775..71b712c3e9 100644
--- a/src/corelib/text/qstring.h
+++ b/src/corelib/text/qstring.h
@@ -36,6 +36,8 @@ Q_FORWARD_DECLARE_CF_TYPE(CFString);
 Q_FORWARD_DECLARE_OBJC_CLASS(NSString);
 #endif
 
+class tst_QString;
+
 QT_BEGIN_NAMESPACE
 
 class QRegularExpression;
@@ -116,6 +118,8 @@ constexpr QChar QAnyStringView::back() const
 class Q_CORE_EXPORT QString
 {
     typedef QTypedArrayData<char16_t> Data;
+
+    friend class ::tst_QString;
 public:
     typedef QStringPrivate DataPointer;
 
@@ -375,6 +379,12 @@ public:
     inline QString &prepend(QLatin1StringView s) { return insert(0, s); }
     QString &prepend(QUtf8StringView s) { return insert(0, s); }
 
+    QString &assign(QAnyStringView s);
+    inline QString &assign(qsizetype n, QChar c)
+    {
+        Q_ASSERT(n >= 0);
+        return fill(c, n);
+    }
     inline QString &operator+=(QChar c) { return append(c); }
 
     inline QString &operator+=(const QString &s) { return append(s); }