How to convert int to QString?

Is there a QString function which takes an int and outputs it as a QString?

Use QString::number():

int i = 42;
QString s = QString::number(i);

And if you want to put it into string within some text context, forget about + operator. Simply do:

// Qt 5 + C++11
auto i = 13;    
auto printable = QStringLiteral("My magic number is %1. That's all!").arg(i);

// Qt 5
int i = 13;    
QString printable = QStringLiteral("My magic number is %1. That's all!").arg(i);

// Qt 4
int i = 13;    
QString printable = QString::fromLatin1("My magic number is %1. That's all!").arg(i);

Moreover to convert whatever you want, you can use QVariant. For an int to a QString you get:

QVariant(3).toString();

A float to a string or a string to a float:

QVariant(3.2).toString();
QVariant("5.2").toFloat();

Yet another option is to use QTextStream and the << operator in much the same way as you would use cout in C++:

QPoint point(5,1);
QString str;
QTextStream(&str) << "Mouse click: (" << point.x() << ", " << point.y() << ").";

// OUTPUT:
// Mouse click: (5, 1).

Because operator <<() has been overloaded, you can use it for multiple types, not just int. QString::arg() is overloaded, for example arg(int a1, int a2), but there is no arg(int a1, QString a2), so using QTextStream() and operator << is convenient when formatting longer strings with mixed types.

Caution: You might be tempted to use the sprintf() facility to mimic C style printf() statements, but it is recommended to use QTextStream or arg() because they support Unicode strings.

I always use QString::setNum().

int i = 10;
double d = 10.75;
QString str;
str.setNum(i);
str.setNum(d);

setNum() is overloaded in many ways. See QString class reference.

In it's simplest form, use the answer of Georg Fritzsche

For a bit advanced, you can use this,

QString QString::arg ( int a, int fieldWidth = 0, int base = 10, const QChar & fillChar = QLatin1Char( ' ' ) ) const

Get the documentation and an example here..

If you need locale-aware number formatting, use QLocale::toString instead.

Just for completeness, you can use the standard library and do QString qstr = QString::fromStdString(std::to_string(42));

QLocale has a handy way of converting numbers.  It's not much more typing than the accepted answer, but is more useful in the case of floats; so I like to do both this way.  Here's for an int:

int i = 42;
QString s = QLocale::toString(i);

and here's for a float:

float f=42.5;
QString s = QLocale::toString(f, 1);

the last argument is the number of decimal places.  You can also insert a char format argument such as 'f' or 'e' for the second parameter. The advantage of this, is if your program is then run in a locale where a comma is used as a decimal "point", it will automatically print it that way.  It's not included in something like <QCoreApplication>, so you'll have to do an #include <QLocale> somewhere, of course.  It really comes into its own in printing out currency strings.

You can write:

QString s = QString::number(42.5, 'f', 1);

but according to the help "Unlike QLocale::toString(), this function does not honor the user's locale settings."