how to initialize a QString to null?

Question

What is the difference between QString::number(0) and ((const char*) 0)?

I want to initialize a QString say phoneNumber to null. Will phoneNumber(QString::number(0)) and phoneNumber((const char*) 0) both work?

Answer 1

To create a null QString just default initialize it:

QString phoneNumber;

// or if you already have a QString variable and want to 'clear' it:

phoneNumber = QString();

Note that QString::number(0) is decidedly not null - it creates a QString with the value "0".

You could also initialize the QString with a NULL pointer, but I wouldn't recommend it unless you're passing a pointer regardless of whether it's NULL or not (i.e., it could sometimes point to a C string) since it's unnecessary.

You should also understand the following Qt docs:

Distinction Between Null and Empty Strings

For historical reasons, QString distinguishes between a null string and an empty string. A null string is a string that is initialized using QString's default constructor or by passing (const char *)0 to the constructor. An empty string is any string with size 0. A null string is always empty, but an empty string isn't necessarily null:

QString().isNull();               // returns true
QString().isEmpty();              // returns true

QString("").isNull();             // returns false
QString("").isEmpty();            // returns true

QString("abc").isNull();          // returns false
QString("abc").isEmpty();         // returns false

All functions except isNull() treat null strings the same as empty strings. For example, toAscii().constData() returns a pointer to a '\0' character for a null string (not a null pointer), and QString() compares equal to QString(""). We recommend that you always use the isEmpty() function and avoid isNull().

Answer 2

#include <QCoreApplication>
#include <QString>
#include <QDebug>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QString name(QString::null);
    // or QString name = QString::null;
    // or QString name;

    qDebug() << name.isNull();
    qDebug() << name;

    return a.exec();
}

Output:

true
""

Michael Burr`s solution is of course also correct... But I like the QString::null more.