External Linkage

The extern keyword is used to declare that the symbol is external to current source file.

Imagine source1.cpp:

int value = 100;

Imagine source2.cpp

void foo () {
    extern int value; // (1)!
    value++;          // (2)!
}

1. Find the value variable outside of source2.cpp.
2. Uses the value variable.

These can be defined within the same source file too.

int value = 100;

void foo () {
    // extern int value;
    value++;
}

The extern declaration in this case becomes optional.