Strange behavior of static global variable

Question

I know that this program is not using the static variable in an appropriate way, but it shows how to reproduce a behavior I have seen :

Main.cpp :

int main(){     MyObject* p = new MyObject();     Header::i = 5;      printf("i %i\n", Header::i);     p->update();      return 0; } 

MyObject.cpp :

MyObject::MyObject(){ }  void MyObject::update(){     printf("i %i\n", Header::i); } 

Extern.h :

namespace Header {     static int i; }; 

The output I get is :

i : 5 i : 0 

Why don't I get 5 for both outputs ? Where does this 0come from ? Could you explain how static variables work ?

Answer 1

Static variables have internal linkage which effectively means they are local to the compilation unit. Since you have the static variable declared in a header included in 2 source files, you basically have 2 distinct variables: one i local to MyObject.cpp and another, different i, local to main.cpp

Answer 2

You have one static variable per translation unit where you include the header, because static variables have internal linkage.

Where does this 0 come from ?

You've not initialized the variable in the second translation unit, and static variables are zero-initialized, that's where the 0 comes from.

In the standard (§3.6.2/2):

Variables with static storage duration (3.7.1) [...] shall be zero-initialized (8.5) before any other initialization takes place.[...]