QString: de-inline toStdString() so we can avoid a QByteArray temporary

We can simply write the UTF-8 string into the std::string buffer,
bypassing our temporary. And when Qt is compiled with C++23, we even get
resize_and_overwrite() support, removing the unnecessary memset() -
though the compiler might have been smart enough to suppress it (GCC and
Clang currently aren't that smart; in fact, GCC even generates code to
deal with a default-constructed std::string not being empty).

Change-Id: I3883268b30a9065757dcfffd6da4705a4aa44d7a
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/corelib/text/qstring.cpp

@@ -9374,6 +9374,26 @@ QString::iterator QString::erase(QString::const_iterator first, QString::const_i
 
     \sa toLatin1(), toUtf8(), toLocal8Bit(), QByteArray::toStdString()
 */
+std::string QString::toStdString() const
+{
+    std::string result;
+    if (isEmpty())
+        return result;
+
+    auto writeToBuffer = [this](char *out, size_t) {
+        char *last = QUtf8::convertFromUnicode(out, *this);
+        return last - out;
+    };
+    size_t maxSize = size() * 3;    // worst case for UTF-8
+#ifdef __cpp_lib_string_resize_and_overwrite
+    // C++23
+    result.resize_and_overwrite(maxSize, writeToBuffer);
+#else
+    result.resize(maxSize);
+    result.resize(writeToBuffer(result.data(), result.size()));
+#endif
+    return result;
+}
 
 /*!
     \fn QString QString::fromRawData(const char16_t *unicode, qsizetype size)

src/corelib/text/qstring.h

@@ -1086,7 +1086,7 @@ public:
     }
 
     static inline QString fromStdString(const std::string &s);
-    inline std::string toStdString() const;
+    std::string toStdString() const;
     static inline QString fromStdWString(const std::wstring &s);
     inline std::wstring toStdWString() const;
 
@@ -1573,9 +1573,6 @@ QT_ASCII_CAST_WARN inline QString operator+(QString &&lhs, const QByteArray &rhs
 #  endif // QT_NO_CAST_FROM_ASCII
 #endif // QT_USE_QSTRINGBUILDER
 
-std::string QString::toStdString() const
-{ return toUtf8().toStdString(); }
-
 QString QString::fromStdString(const std::string &s)
 { return fromUtf8(s.data(), qsizetype(s.size())); }
 

src/corelib/text/qstringconverter.cpp

@@ -691,6 +691,14 @@ char *QUtf8::convertFromUnicode(char *out, QStringView in, OnErrorLambda &&onErr
     return reinterpret_cast<char *>(dst);
 }
 
+char *QUtf8::convertFromUnicode(char *dst, QStringView in) noexcept
+{
+    return convertFromUnicode(dst, in, [](auto *dst, ...) {
+        // encoding error - append '?'
+        *dst++ = '?';
+    });
+}
+
 QByteArray QUtf8::convertFromUnicode(QStringView in)
 {
     qsizetype len = in.size();
@@ -698,11 +706,7 @@ QByteArray QUtf8::convertFromUnicode(QStringView in)
     // create a QByteArray with the worst case scenario size
     QByteArray result(len * 3, Qt::Uninitialized);
     char *dst = const_cast<char *>(result.constData());
-    dst = convertFromUnicode(dst, in, [](auto *dst, ...) {
+    dst = convertFromUnicode(dst, in);
-        // encoding error - append '?'
-        *dst++ = '?';
-    });
-
     result.truncate(dst - result.constData());
     return result;
 }

src/corelib/text/qstringconverter_p.h

@@ -320,6 +320,7 @@ struct QUtf8
 
     static char16_t *convertToUnicode(char16_t *dst, QByteArrayView in, QStringConverter::State *state);
 
+    static char *convertFromUnicode(char *dst, QStringView in) noexcept;
     Q_CORE_EXPORT static QByteArray convertFromUnicode(QStringView in);
     Q_CORE_EXPORT static QByteArray convertFromUnicode(QStringView in, QStringConverter::State *state);
     static char *convertFromUnicode(char *out, QStringView in, QStringConverter::State *state);