Porting to Qt 4

QString

Here are the main issues to be aware of when porting QString to Qt 4:

1. The QString::QString(QChar) constructor performed implicit conversion in Qt 3. Now, you will need a cast to convert a QChar to a QString.
2. The QString::QString(const QByteArray &) constructor used to stop at the first '\0' it encountered, for compatibility with Qt 1. This quirk has now been fixed; in Qt 4, the resulting QString always has the same length as the QByteArray that was passed to the constructor.
3. The QString::null static constant has been deprecated in Qt 4. For compatibility, Qt 4 provides a QString::null symbol that behaves more or less the same as the old constant. The new idiom is to write QString() instead of QString::null, or to call clear().

   For example, if you have code like

            str1 = QString::null;
            if (str2 == QString::null)
                do_something(QString::null);

   you can rewrite it as

            str1.clear();
            if (str2.isNull())
                do_something(QString());

   In new code, we recommend that you don't rely on the distinction between a null string and a (non-null) empty string. See Distinction Between Null and Empty Strings for details.

4. QString::latin1() and QString::ascii() have been replaced with QString::toLatin1() and QString::toAscii(), which return a QByteArray instead of a (non-reentrant) const char *. For consistency, QString::utf8() and QString::local8Bit(), which already returned a QByteArray (actually a QCString), have been renamed QString::toUtf8() and QString::toLocal8Bit().

   To obtain a const char * pointer to ASCII or Latin-1 data, use QString::toAscii() or QString::toLatin1() to obtain a QByteArray containing the data, then call QByteArray::constData() to access the character data directly. Note that the pointer returned by this function is only valid for the lifetime of the byte array; you should avoid taking a pointer to the data contained in temporary objects.

        QString greeting = "Hello";
        const char *badData = greeting.toAscii().constData(); // data is invalid
        QByteArray asciiData = greeting.toAscii();
        const char *goodData = asciiData.constData();

   In the above example, the goodData pointer is valid for the lifetime of the asciiData byte array. If you need to keep a copy of the data in a non-Qt data structure, use standard C memory allocation and string copying functions to do so before destroying the byte array.

5. QString::at() returned a non-const reference, whereas the new QString::at() returns a const value. Code like

        str.at(0) = 'X';

   will no longer compile. Instead, use QString::operator[]:

        str[0] = 'X';

6. The QString::contains(x) function (where x is a character or a string) has been renamed QString::count(x). In addition, there now exists a set of QString::contains() functions that returns a boolean value. Replace old calls to contains() with either count() or contains(), depending on whether you care about the specific number of occurrences of a character in the string or only care about whether the string contains that character or not.
7. Many functions in QString had a bool parameter that specified case sensitivity. In Qt 4, in the interest of code readability and maintainability, the bool parameters have been replaced by the Qt::CaseSensitivity enum, which can take the values Qt::CaseSensitive and Qt::CaseInsensitive.

   For example, if you have code like

            if (url.startsWith("http:", false))
                ...

   you can rewrite it as

            if (url.startsWith("http:", Qt::CaseInsensitive))
                ...

8. The QString::setExpand(uint, QChar) function, which already was obsolete in Qt 3, is no longer available. Use QString::operator[] instead.

   For example, if you have code like

            str.setExpand(32, '$');

   you can rewrite it as

            str[32] = '$';

9. The QT_NO_ASCII_CAST and QT_NO_CAST_ASCII macros have been renamed QT_NO_CAST_TO_ASCII and QT_NO_CAST_FROM_ASCII, respectively.
10. The QString::data() used to return the same as QString::ascii(). It now returns a pointer to the Unicode data stored in the QString object. Call QString::ascii() if you want the old behavior.
11. QString::arg() now converts two-digit place markers, allowing up to 99 place markers to be used in any given string.
12. Comparisons between QStrings and NULL in order to determine whether strings are empty are no longer allowed. Use isEmpty() instead.

 QVBox *vbox = new QVBox;
 QPushButton *child1 = new QPushButton(vbox);
 QPushButton *child2 = new QPushButton(vbox);

 QWidget *vbox = new QWidget;
 QPushButton *child1 = new QPushButton;
 QPushButton *child2 = new QPushButton;

 QVBoxLayout *layout = new QVBoxLayout;
 layout->addWidget(child1);
 layout->addWidget(child2);
 vbox->setLayout(layout);