Question

Suppose you have a static global variable in your header file, and you use this variable in your main.cpp.

// header.h
static int variableOne = 100;

//main.cpp
   .
   .
   cout << variableOne << endl;

Will main.cpp get its own copy of variableOne (although the value is still 100…)? Or am I mixing this concept with extern (I know that extern tells the compiler that variableOne is defined elsewhere in the project…)

Thank you.

Answer 1

1. What does it mean #include

When you write #include "header.h" the compiler is basically just copying the whole content of the file instead of that line. In C++ an header file has no semantic meaning, it’s just used for textual substitution.

So if you have

---- header.h ----
static int variableOne = 100;

---- main.cpp ----
#include "header.h"

...
std::cout << variableOne << std::endl;

for the compiler is exactly the same as

---- main.cpp ----
static int variableOne = 100;

...
std::cout << variableOne << std::endl;

2. What does it mean static in that context

A static variable is like a global for the lifetime (i.e. it’s constructed before main starts and it’s destroyed after main terminates) but is only visible inside the translation unit that defines it. So you can have the same static variable name used in different translation units and all those variables are distinct (each translation unit will see its own static variable).

3. What happens when you put a static variable declaration in an header?

The net result is that every translation unit that will include that header will get its own static variable with that name because it’s exactly the same as if the translation unit defined a static variable in the .cpp and not in the header file.

The name will be the same for all of them but they will be different variables. Of course if the header file declares and initializes the variable then the initial value will be the same, but all those variables will be distinct and if for example one translation unit changes that variable, the change will not be seen by other translation units.