Static variable in a Header File

Question

Static variable has file scope. Say I have two following files:

• file1.h
• file1.cpp
• file2.h
• file2.cpp

I have declared static variable say static int Var1 in both the header files. Both file1.h and file2.h are included in main.cpp file.

I did this since the static variable will have file scope so it won't conflict each other. But after compilation I found it is showing conflict.

Now static variable is behaving like a extern variable. On the other hand if I declare the static variable in both .cpp files, it compiles well.

I am unable to understand this behavior.

Can any body explain how scope and linkage are working in this scenario.

Answer 1

Static variables have translation unit scope (usually a .c or .cpp file), but an #include directive simply copies the text of a file verbatim, and does not create another translation unit. After preprocessing, this:

#include "file1.h"
#include "file2.h"

Will turn into this:

/* file1.h contents */
static int Var1;

/* file2.h contents */
static int Var1;

Which, as you know, is invalid.