Using export USER=myName doesn't change the output of whoami

I saw this example $ whoami MatsP $ g++ -Wall -std=c++0x getenv.cpp $ ./a.out MatsP $ export USER=DonaldDuck $ ./a.out DonaldDuck but for me it doesn't work. I am in a Debian machine. I used export -p and indeed the variable changed. I refreshed the bachrc, still nothing. How to achiece the above? whoami displays the default value.