Introduce a new class QStringBuilder to speed up the creation of

QString objects from smaller chunks.

The QStringBuilder class:

QStringBuilder uses expression templates (using the '%' operator)
to postpone any actual concatenation until it is assigned to an
actual QString. At that time it knows the exact sizes of all chunks,
can compute the required space, allocates once a QString of
appriopriate size and then copies over the chunk data one-by-one.

In addition, QLatin1Literal is a drop-in replacement for QLatin1String
(which we can't change for compatibility reasons) that knows its
size, therefore saving a few cycles when computing the size of the
resulting string.

Some further saved cycles stem from inlining and reduced reference
counting logic (the QString created from a QStringBuilder has typically
ref count equal to 1, while QString::append() needs an extra test)

Minor changes to the existing QString class:

- Introduce QString constructor to create an uninitialized QString of a given size.
This particular constructor is used by QStringBuilder class.

- Introduce a QT_USE_FAST_CONCATENATION macro to disable the existing
overloads of operator+() and helps finding the places where they are used in code.

- Introduce QT_USE_FAST_OPERATOR_PLUS.  This also disables the existing
overloads of operator+() and creates a new templated operator+() with
identical implementation of operator%().  This allows code that is compilable
QT_CAST_{TO,FROM}_ASCII to use QStringBuilder almost transparently. The only
case that is not covered is creating objects like QUrl that are implicitly
constructible from a QString from a QStringBuilder result.  This needs to be
converted explicitly to a QString first, e.g. by using QUrl
url(QString(QLatin1String("http://") + hostName));

Reviewed-by: MariusSO

1 files changed, 12 insertions, 3 deletions

diff --git a/src/corelib/tools/qstring.h b/src/corelib/tools/qstring.h
index 69c4f2f..7c0d6a3 100644
--- a/src/corelib/tools/qstring.h
+++ b/src/corelib/tools/qstring.h
@@ -579,6 +579,9 @@ public:
     bool isSimpleText() const { if (!d->clean) updateProperties(); return d->simpletext; }
     bool isRightToLeft() const { if (!d->clean) updateProperties(); return d->righttoleft; }
 
+    struct Uninitialized {};
+    QString(int size, Uninitialized);
+
 private:
 #if defined(QT_NO_CAST_FROM_ASCII) && !defined(Q_NO_DECLARED_NOT_DEFINED)
     QString &operator+=(const char *s);
@@ -1001,13 +1004,15 @@ inline int QByteArray::findRev(const QString &s, int from) const
 #  endif // QT3_SUPPORT
 #endif // QT_NO_CAST_TO_ASCII
 
+#ifndef QT_USE_FAST_OPERATOR_PLUS
+# ifndef QT_USE_FAST_CONCATENATION
 inline const QString operator+(const QString &s1, const QString &s2)
 { QString t(s1); t += s2; return t; }
 inline const QString operator+(const QString &s1, QChar s2)
 { QString t(s1); t += s2; return t; }
 inline const QString operator+(QChar s1, const QString &s2)
 { QString t(s1); t += s2; return t; }
-#ifndef QT_NO_CAST_FROM_ASCII
+#  ifndef QT_NO_CAST_FROM_ASCII
 inline QT_ASCII_CAST_WARN const QString operator+(const QString &s1, const char *s2)
 { QString t(s1); t += QString::fromAscii(s2); return t; }
 inline QT_ASCII_CAST_WARN const QString operator+(const char *s1, const QString &s2)
@@ -1020,7 +1025,9 @@ inline QT_ASCII_CAST_WARN const QString operator+(const QByteArray &ba, const QS
 { QString t = QString::fromAscii(ba.constData(), qstrnlen(ba.constData(), ba.size())); t += s; return t; }
 inline QT_ASCII_CAST_WARN const QString operator+(const QString &s, const QByteArray &ba)
 { QString t(s); t += QString::fromAscii(ba.constData(), qstrnlen(ba.constData(), ba.size())); return t; }
-#endif
+#  endif // QT_NO_CAST_FROM_ASCII
+# endif // QT_USE_FAST_CONCATENATION
+#endif // QT_USE_FAST_OPERATOR_PLUS
 
 #ifndef QT_NO_STL
 inline std::string QString::toStdString() const
@@ -1229,6 +1236,8 @@ inline int QStringRef::localeAwareCompare(const QStringRef &s1, const QStringRef
 
 QT_END_NAMESPACE
 
-QT_END_HEADER
+#ifdef QT_USE_FAST_CONCATENATION
+#include <QtCore/qstringbuilder.h>
+#endif
 
 #endif // QSTRING_H