including header files in .cpp files

That is a linking error and means that you are not including a sourcefile that has the definition for SpreadSheetCell:SpreadSheetCell() when compiling.

You are probably missing SpreadSheetCell.cpp in the command line you use, e.g. something like:

g++ -o myBinary SpreadSheet.cpp SpreadSheetCell.cpp [...more files?]

It's never a good idea to include source code. Always only include header files.

I can't look in your file(s), however, I think that SPreadSheetCell.h includes a file (SpreadSheet.h maybe?) so that a circular dependency exist.

The best way is to make a graph of the files that are included (in both cpp and h files). If there is a 'circle' somewhere you have found the problem. In that case you should remove one link so the circle is broken or split files up in smaller files (also to remove the circle).

(with 'circle' I mean e.g. a file A including B including C including A again.)

Ensure your SpreadSheetCell.cpp has a #include "SpreadSheetCell.h", if not show your code and your compiling commands!