extern in C/C++

How exactly to use extern in C and C++ ?

The extern keyword is used to tell the compiler that a data object is declared in a different *.cpp or *.c file (code unit). Its required for data objects but optional for function declarations. For example, you have two *.cpp files named A.cpp and B.cpp. B.cpp has a global int that needs to be used in A.cpp.

// A.cpp
#include
// other includes here
...
extern int hours; // this is declared globally in B.cpp

int foo()
{
hours = 1;
}



// B.cpp
#include
// other includes here
...
int hours; // here we declare the object WITHOUT extern
extern void foo(); // extern is optional on this line

int main()
{
foo();
}