problem with linking with QtCreator "error: collect2: ld..."

I am trying to link some .h files but there is the following error:

Code:

:-1: error: collect2: ld returned 1 exit status

followed by notes like these:

Code:

/home/alex/Qt/test2/main.cpp:40: undefined reference to `LCDRange::setValue(int)'
/home/alex/Qt/test2/main.cpp:26: undefined reference to `CannonField::CannonField(QWidget*)'

I cannot understand why this is happening.....
Somebody else had the same problem and a recommended solution was to install either libgd2-noxpm either libgd2-xpm either libgd2-noxpm-dev or libgd...
The other user tried to do so with no good result. So did I...
Can anyone help me?
i use Ubuntu 9.10 and using
Qt Creator 1.2.1
Based on Qt 4.5.2 (32 bit)

Built on Sep 1 2009 at 17:30:22

Those methods are not implemented in your files. The compiler compiled them as references to code implemented elsewhere, but when the linker went looking for the implementation, it couldn't find them.