Question

QDir *temp = new QDir("%USERPROFILE%");
bool ok = temp->mkdir("abc");

and it does not work.

Answer 1

For portable Qt code, use static method QProcessEnvironment::systemEnvironment():

QProcessEnvironment env(QProcessEnvironment::systemEnvironment());
QDir *temp = new QDir(env.value("USERPROFILE")); // returns empty string for unset variable
bool ok = temp->mkdir("abc");

Alternative, as suggested in a comment of the actual question, you can also use

#include <cstdio>

QString envValue(QString::fromLocal8bit(::getenv("USERPROFILE"))); // standard, yet deprecated in Windows...

I’d prefer using the first alternative, letting Qt take care of character encoding stuff. If you want to use non-Qt functions, I think in Windows you should use getenv_s or _wgetenv_s to do this “properly”.