Namely,when I use keyword "extern" to get the value of standard variable (e.g.float,int,..) or array located in one file and use it in another file, everything works fine.
Problems appear when I recall to matrix variable (I use matrix.h library from http://www.techsoftpl.com/matrix/matlite.htm)then I got this message:
[Linker Warning] Public symbol '_slupek' defined in both module C:\PROGRAM FILES\BORLAND\CBUILDER6\PROJECTS\05.11.2009\SLUPKI.OBJ and C:\PROGRAM FILES\BORLAND\CBUILDER6\PROJECTS\05.11.2009\KATY.OBJ
Below are listings maybe someone will find mistake.
When I put extern double x_1[6] and extern double x_2[6] above katy() function in katy.cpp additionally I slash out extern Matrix slupek(6,2) and replace respectively slupek(i,0) and slupek(i,1) with x_1[i] and x_2[i] my program works fine.
I got confused with slupki (which is a function) and slupek (which is a variable).
So ignore what I said in the previous posts.
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So the problem is with the declaration of the slupek variable.
This Matrix slupek(6,2); and this extern Matrix slupek(6,2); are both the same
in that they are both definitions
One should be Matrix slupek(6,2); and the other should be extern Matrix slupek;.
So pretty much as JSmith said.