C++ Handbook

`extern` — C++ Keyword

`extern` — C++ Keyword

The extern keyword in C++: declares a variable or function defined in another translation unit.

How to use this reference page

Use reference pages to confirm names, categories, nearby facilities, and the constraints that matter before writing or reviewing code.

  • Scan the top of the page first to identify the primary types, functions, or algorithm families involved.
  • Use the nearby-page links when your question is really about a companion header, related algorithm family, or broader subsystem.
  • Validate tricky behavior with a small compileable example before relying on memory for details like invalidation, ordering, allocation, or lifetime rules.

extern

Declares that a variable or function is defined elsewhere (another translation unit or a C library). Also used with a string literal for language linkage (extern "C").

Syntax

extern Type name;                  // declaration (no definition)
extern Type name = value;          // definition (extern is redundant here)
extern "C" return-type func();     // C language linkage
extern "C" { /* declarations */ }

Example

// --- config.cpp ---
// int g_timeout = 30;   // definition lives here

// --- main.cpp ---
#include <print>

extern int g_timeout;  // declaration: defined in config.cpp

// Calling a C library function
extern "C" {
    int c_add(int a, int b);  // hypothetical C function
}

int main() {
    // std::println("{}", g_timeout);   // uses definition from config.cpp
    (void)g_timeout;

    // std::println("{}", c_add(2, 3)); // calls C function without name mangling
}

Notes

Example in practice

int main() {
    // Pick one facility from this reference page.
    // Write the smallest program that exercises its main precondition,
    // complexity rule, or lifetime constraint before scaling up.
    return 0;
}