Undefined reference to when including file from separate directory-Collection of common programming errors

I have very basic QT application (just create to explain my problem). So here I go 🙂 I have two folders, f1 and f2, and they are on same level (have same folder for parent). In f1 I have source code from my project, and in f2 another project. For sake of this example, let’s say that in f1 I have only common.h and common.cpp, and in f2 I have:

Test.pro

SOURCES = main.cpp
INCLUDEPATH += "..//f1//"

main.cpp

#include 
#include 

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

    qDebug()

Originally posted 2013-11-27 12:26:46.