extern

The extern storage class specifier can modify a declaration in one of the three following ways, depending on context:

It can be used to declare a variable without defining it. Typically, this is used in a header file for a variable that will be defined in a separate implementation file.

// global scope
int x;             // definition; x will be default-initialized
extern int y;      // declaration; y is defined elsewhere, most likely another TU
extern int z = 42; // definition; "extern" has no effect here (compiler may warn)

It gives external linkage to a variable at namespace scope even if const or constexpr would have otherwise caused it to have internal linkage.

// global scope
const int w = 42;            // internal linkage in C++; external linkage in C
static const int x = 42;     // internal linkage in both C++ and C
extern const int y = 42;     // external linkage in both C++ and C
namespace {
    extern const int z = 42; // however, this has internal linkage since
                             // it's in an unnamed namespace
}

It redeclares a variable at block scope if it was previously declared with linkage. Otherwise, it declares a new variable with linkage, which is a member of the nearest enclosing namespace.

// global scope
namespace {
    int x = 1;
    struct C {
        int x = 2;
        void f() {
            extern int x;           // redeclares namespace-scope x
            std::cout << x << '\n'; // therefore, this prints 1, not 2
        }
    };
}
void g() {
    extern int y; // y has external linkage; refers to global y defined elsewhere
}

A function can also be declared extern, but this has no effect. It is usually used as a hint to the reader that a function declared here is defined in another translation unit. For example:

void f();        // typically a forward declaration; f defined later in this TU
extern void g(); // typically not a forward declaration; g defined in another TU

In the above code, if f were changed to extern and g to non-extern, it would not affect the correctness or semantics of the program at all, but would likely confuse the reader of the code.

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