Introduce a new QStringBuilder helper class to speed up building strings

from smaller chunks using expression templates.

Simple test and benchmark included, the class is not yet complete.

src/libs/utils/qstringbuilder.cpp

+/****************************************************************************
+**
+** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
+** Contact: Qt Software Information (qt-info@nokia.com)
+**
+** This file is part of the $MODULE$ of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial Usage
+** Licensees holding valid Qt Commercial licenses may use this file in
+** accordance with the Qt Commercial License Agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and Nokia.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 2.1 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL included in the
+** packaging of this file.  Please review the following information to
+** ensure the GNU Lesser General Public License version 2.1 requirements
+** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** In addition, as a special exception, Nokia gives you certain
+** additional rights. These rights are described in the Nokia Qt LGPL
+** Exception version 1.0, included in the file LGPL_EXCEPTION.txt in this
+** package.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3.0 as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL included in the
+** packaging of this file.  Please review the following information to
+** ensure the GNU General Public License version 3.0 requirements will be
+** met: http://www.gnu.org/copyleft/gpl.html.
+**
+** If you are unsure which license is appropriate for your use, please
+** contact the sales department at qt-sales@nokia.com.
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include "qstringbuilder.h"
+
+/*!
+    \class QLatin1Literal
+    \reentrant
+
+    \brief The QLatin1Literal class provides a thin wrapper of string literal
+    used in source codes.
+
+    The main use of \c QLatin1Literal is in conjunction with \c QStringBuilder
+    to reduce the number of reallocations needed to build up a string from
+    smaller chunks.
+
+    Contrary to \c QLatin1String, a \c QLatin1Literal can retrieve its size
+    int \i{O(1)}.
+
+    \ingroup tools
+    \ingroup shared
+    \ingroup text
+    \mainclass
+
+    \sa QStringBuilder, QLatin1String, QString
+
+*/
+
+/*! \fn QLatin1Literal::QLatin1Literal(const char(&literal)[])
+ 
+    The only constructor of the class. 
+*/
+
+/*! \fn int QLatin1Literal::size() const
+ 
+    Returns the number of characters in the literal \i{not} including
+    the trailing NUL char.
+*/
+
+/*! \fn char *QLatin1Literal::size() const
+ 
+    Returns a pointer to the first character of the string literal.
+    The string literal is terminated by a NUL character.
+*/
+
+/*! \fn QLatin1Literal::operator QString() const
+ 
+    Converts the \c QLatin1Literal into a \c QString object.
+*/
+
+
+/*!
+    \class QStringBuilderPair
+    \reentrant
+
+    \brief QStringBuilderPair is a helper class template for building
+    QStringBuilder objects wrapping two smaller QStringBuilder object.
+*/
+
+
+/*!
+    \class QStringBuilder
+    \reentrant
+
+    \brief QStringBuilder is a template class providing a facility to build
+    up QStrings from smaller chunks.
+
+    \ingroup tools
+    \ingroup shared
+    \ingroup text
+    \mainclass
+
+    When creating strings from smaller chunks, typically \c QString::operator+()
+    is used, resulting \i{n - 1} reallocations when operating on \i{n} chunks.
+
+    QStringBuilder uses expression
+    templates to collect the individual parts, compute the total size,
+    allocate memory for the resulting QString object, and copy the contents
+    of the chunks into the result.
+
+    Using \c QStringBuilder::operator%() yield generally better performance then
+    using \c QString::operator+() on the same chunks if there are three or
+    more of them, and equal performance otherwise.
+
+    \sa QLatin1Literal, QString
+*/
+
+/* !fn template <class A, class B> QStringBuilder< QStringBuilderPair<A, B> > operator%(const A &a, const B &b)
+ 
+   Creates a helper object containing both parameters. 
+
+   This is the main function to build up 
+*/

src/libs/utils/qstringbuilder.h

+/****************************************************************************
+**
+** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
+** Contact: Qt Software Information (qt-info@nokia.com)
+**
+** This file is part of the $MODULE$ of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial Usage
+** Licensees holding valid Qt Commercial licenses may use this file in
+** accordance with the Qt Commercial License Agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and Nokia.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 2.1 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL included in the
+** packaging of this file.  Please review the following information to
+** ensure the GNU Lesser General Public License version 2.1 requirements
+** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** In addition, as a special exception, Nokia gives you certain
+** additional rights. These rights are described in the Nokia Qt LGPL
+** Exception version 1.0, included in the file LGPL_EXCEPTION.txt in this
+** package.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3.0 as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL included in the
+** packaging of this file.  Please review the following information to
+** ensure the GNU General Public License version 3.0 requirements will be
+** met: http://www.gnu.org/copyleft/gpl.html.
+**
+** If you are unsure which license is appropriate for your use, please
+** contact the sales department at qt-sales@nokia.com.
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#ifndef QSTRINGBUILDER_H
+#define QSTRINGBUILDER_H
+
+#include <QtCore/qstring.h>
+
+#include <string.h>
+
+class QLatin1Literal
+{
+public:
+    template <int N>
+    QLatin1Literal(const char (&str)[N]) : m_size(N - 1), m_data(str) {}
+
+    int size() const { return m_size; }
+    const char *data() const { return m_data; }
+
+    void append(QChar *&out) const
+    {
+        const char *s = m_data;
+        for (int i = m_size; --i >= 0;)    
+            *out++ = *s++;
+    }
+
+    operator QString() const
+    {
+        QString s;
+        s.resize(m_size);
+        QChar *d = s.data();
+        append(d);
+        return s;
+    }
+
+private:
+    const int m_size;
+    const char *m_data;
+};
+
+template <class A>
+class QStringBuilder : public A
+{
+public:
+    QStringBuilder(A a_) : A(a_) {}
+
+    operator QString() const
+    {
+        QString s;
+        s.resize(this->size());
+        QChar *d = s.data();
+        this->append(d);
+        return s;
+    }
+
+};
+
+template <>
+class QStringBuilder<QString>
+{
+public:
+    QStringBuilder(const QString &a_) : a(&a_) {}
+
+    inline int size() const { return a->size(); }
+
+    inline void append(QChar *&out) const
+    {
+        const int n = a->size();
+        memcpy(out, (char*)a->constData(), sizeof(QChar) * n);
+        out += n; 
+    }
+
+    inline operator QString() const { return *a; }
+
+private:
+    const QString *a;
+};
+
+template <class A, class B>
+class QStringBuilderPair
+{
+public:
+    QStringBuilderPair(A a_, B b_) : a(a_), b(b_) {}
+    inline int size() const { return a.size() + b.size(); }
+    inline void append(QChar *&out) const { a.append(out); b.append(out); }
+private:
+    A a;
+    B b;
+};
+
+
+template <class A, class B>
+QStringBuilder< QStringBuilderPair<A, B> >
+operator%(const A &a, const B &b)
+{
+    return QStringBuilderPair<A, B> (a, b);
+}
+
+template <class A>
+inline QStringBuilder< QStringBuilderPair<A, QStringBuilder<QString> > >
+operator%(const A &a, const QString &b)
+{
+    return QStringBuilderPair<A, QStringBuilder<QString> > (a, b);
+}
+
+template <class B>
+inline QStringBuilder< QStringBuilderPair<QStringBuilder<QString>, B> >
+operator%(const QString &a, const B &b)
+{
+    return QStringBuilderPair<QStringBuilder<QString>, B> (a, b);
+}
+
+
+inline QStringBuilder<
+    QStringBuilderPair<QStringBuilder<QString>,
+    QStringBuilder<QString> >
+>
+operator%(const QString &a, const QString &b)
+{
+    return QStringBuilderPair< QStringBuilder<QString>,
+        QStringBuilder<QString> > (a, b);
+}
+
+#endif // QSTRINGBUILDER_H

tests/auto/qstringbuilder/qstringbuilder.pro

+load(qttest_p4)
+SOURCES  += tst_qstringbuilder.cpp
+
+QT -= gui
+
+QT = core
+
+DEFINES += QT_NO_CAST_TO_ASCII
+
+
+

tests/auto/qstringbuilder/tst_qstringbuilder.cpp

+/****************************************************************************
+**
+** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
+** Contact: Qt Software Information (qt-info@nokia.com)
+**
+****************************************************************************/
+
+#include <QtTest/QtTest>
+#include "../../../src/libs/utils/qstringbuilder.h"
+
+//TESTED_CLASS=QStringBuilder
+//TESTED_FILES=qstringbuilder.h
+
+class tst_QStringBuilder : public QObject
+{
+    Q_OBJECT
+
+public:
+    tst_QStringBuilder();
+    ~tst_QStringBuilder() {}
+
+public slots:
+    void init() {}
+    void cleanup() {}
+
+private slots:
+    void operator_percent();
+};
+
+
+tst_QStringBuilder::tst_QStringBuilder()
+{
+    //QTextCodec::setCodecForLocale(QTextCodec::codecForName("ISO 8859-1"));
+}
+
+void tst_QStringBuilder::operator_percent()
+{
+    QLatin1Literal l1literal("a literal");
+    QLatin1String l1string("a literal");
+    QLatin1Char l1char('c');
+    QChar qchar(l1char);
+
+    QCOMPARE(QString(l1literal % l1literal), QString(l1string + l1string));
+}
+
+QTEST_APPLESS_MAIN(tst_QStringBuilder)
+
+#include "tst_qstringbuilder.moc"

tests/benchmarks/qstringbuilder/main.cpp

+
+#include <QDebug>
+#include <QString>
+
+#include "../../../src/libs/utils/qstringbuilder.h"
+
+#include <qtest.h>
+
+
+class tst_qstringbuilder : public QObject
+{
+    Q_OBJECT
+
+public:
+    tst_qstringbuilder();
+
+private slots:
+    // QStringBuilder based 
+    void builderbased_l1literal_l1literal();
+    void builderbased_l1literal_l1literal_l1literal();
+    void builderbased_l1literal_l1literal_l1literal_l1literal();
+
+    // QString based for comparison
+    void stringbased_l1string_l1string();
+    void stringbased_l1string_l1string_l1string();
+    void stringbased_l1string_l1string_l1string_l1string();
+
+private:
+    const QLatin1Literal l1literal;
+    const QLatin1String l1string;
+};
+
+
+tst_qstringbuilder::tst_qstringbuilder()
+    : l1literal("some literal"), l1string("some literal")
+{}
+
+void tst_qstringbuilder::builderbased_l1literal_l1literal()
+{
+    QString result;
+    QBENCHMARK { result = l1literal % l1literal; }
+    QCOMPARE(result, l1string + l1string);
+}
+
+void tst_qstringbuilder::builderbased_l1literal_l1literal_l1literal()
+{
+    QString result;
+    QBENCHMARK { result = l1literal % l1literal % l1literal; }
+    QCOMPARE(result, l1string + l1string + l1string);
+}
+
+void tst_qstringbuilder::builderbased_l1literal_l1literal_l1literal_l1literal()
+{
+    QString result;
+    QBENCHMARK { result = l1literal % l1literal % l1literal % l1literal; }
+    QCOMPARE(result, l1string + l1string + l1string + l1string);
+}
+
+
+
+void tst_qstringbuilder::stringbased_l1string_l1string()
+{
+    QString result;
+    QBENCHMARK { result = l1string + l1string; }
+    QCOMPARE(result, QString(l1literal % l1literal));
+}
+
+void tst_qstringbuilder::stringbased_l1string_l1string_l1string()
+{
+    QString result;
+    QBENCHMARK { result = l1string + l1string + l1string; }
+    QCOMPARE(result, QString(l1literal % l1literal % l1literal));
+}
+
+void tst_qstringbuilder::stringbased_l1string_l1string_l1string_l1string()
+{
+    QString result;
+    QBENCHMARK { result = l1string + l1string + l1string + l1string; }
+    QCOMPARE(result, QString(l1literal % l1literal % l1literal % l1literal));
+}
+
+QTEST_MAIN(tst_qstringbuilder)
+
+#include "main.moc"

tests/benchmarks/qstringbuilder/qstringbuilder.pro

+load(qttest_p4)
+TEMPLATE = app
+TARGET = tst_qstringbuilder
+DEPENDPATH += .
+INCLUDEPATH += .
+
+QT -= gui
+
+CONFIG += release
+
+# Input
+SOURCES += main.cpp