allAboutSeparateCompilation.txt

As we can see, neither drawTexasFlag.cpp nor drawings.cpp
is "complete" on its own:


> cd lab04
> ls
Makefile            drawNightFlags.sh  drawTexasFlag.sh  drawings.h
drawNightFlags.cpp  drawTexasFlag.cpp  drawings.cpp
>

As you can see, we only have .cpp and .h and files, and a Makefile 
in this directory at the moment.


> CC drawTexasFlag.cpp
Undefined                       first referenced
 symbol                             in file
void writeGnuplotFile(const char*const,const char*const,const char*const,double,double)  drawTexasFlag.o
void drawTexasFlag(std::ostream &,double,double,double) drawTexasFlag.o
ld: fatal: Symbol referencing errors. No output written to a.out
>  

drawTexasFlag.cpp "needs" a couple of functions it doesn't have


> CC drawings.cpp
Undefined                       first referenced
 symbol                             in file
main                                /opt/SUNWspro/prod/lib/crt1.o
ld: fatal: Symbol referencing errors. No output written to a.out
>

drawings.cpp needs a main to be complete.  We need to bring
the two of these together.  Each has what the other needs.
A kind of geek love story.

So now we compile each, "without linking".   The -c flag on the compiler
means "compile, but don't link".  In each case, the output will
be a _____________________ not an exectuable file.

> CC -c drawTexasFlag.cpp
> CC -c drawings.cpp
> 

The answer to fill in the blank is a ".o" file (pronounced "dot oh".)

For example, the command 

> CC -c drawTexasFlag.cpp


produces the file drawTexasFlag.o

and the command 


> CC -c drawings.cpp

produces the command drawings.o


To link the two together into an executable file, you use the command:

CC drawings.o drawTexasFlag.o -o drawTexasFlag

The order of the .o files is not important, but "drawTexasFlag" has to
come immediately after -o.  

The name of the executable comes from the
file that contains the main.  So in this case, since the main function
was in drawTexasFlag.cpp, we use "-o drawTexasFlag" to get an exectuable
named "drawTexasFlag".