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Next, you need to learn how to distribute global-variable declarations and global-variable definitions among header files and source-code files.
As explained in Segment 129, when you define a global variable, you tell the compiler to allocate a chunk of memory for the variable. Consider the following definition, for example:
int mode;
Such a definition, appearing outside of any function definition, tells the
C++ compiler to allocate storage for the mode variable.
If you wish to make use of the mode global variable in another file,
you must so inform the C++ compiler by inserting a declaration in that
other file. When you
declare a global variable, you tell the
compiler that the global variable is defined in an object file
available to the C++ linker when it is time to link together object files
into an executable program.