Qt QString.arg() not working [duplicate] - c++

Question

This question already has answers here:
How to deal with "%1" in the argument of QString::arg()?
(3 answers)
Closed 9 years ago.
I have tried to use the example given in the Qt4.8 documentation:
#include <QCoreApplication>
#include <QString>
#include <iostream>
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
QString str;
str = "%1 %2";
str.arg("%1f", "Hello"); // returns "%1f Hello"
std::cout << str.toStdString().c_str() << std::endl;
str.arg("%1f").arg("Hello"); // returns "Hellof %2"
std::cout << str.toStdString().c_str() << std::endl;
return a.exec();
}
However this outputs :
%1 %2
%1 %2
both times. I have tried this on Windows 7 and Ubuntu, using QtCreator and from the command line. I have checked I have
QMake version 2.01a
Using Qt version 4.8.1 in /usr/lib/x86_64-linux-gnu
and in Windows:
QMake version 2.01a
Using Qt version 4.7.0 in C:\qt\4.7.0\lib
I have even checked my source files for non-ascii characters, e.g. the "%" sign is correct. Please tell me why this doesn't work!
Here is the PRO file I am using:
QT += core
QT -= gui
TARGET = testarg
CONFIG += console
CONFIG -= app_bundle
TEMPLATE = app
SOURCES += main.cpp

Answer 1

The arg() functions do not modify the QString (if you look at the docs, these functions are const.) They return a new QString instead, which you aren't saving anywhere. If you want to modify the original string, you can do so with:
str = str.arg("%1f", "Hello");
If you want to preserve the original, just use a new QString instead:
QString tmp = str.arg("%1f", "Hello");

Answer 2

#include <QCoreApplication>
#include <QString>
#include <iostream>
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
QString str;
str = "%1 %2";
QString a = str.arg("%1f", "Hello"); // returns "%1f Hello"
std::cout << a.toStdString().c_str() << std::endl;
QString b = str.arg("%1f").arg("Hello"); // returns "Hellof %2"
std::cout << b.toStdString().c_str() << std::endl;
return a.exec();
}
note all arg overloads are const and return QString :).