Using the extern Keyword

The extern keyword in C++ is used to declare a variable or function that is defined in another translation unit.

This is particularly useful when working with global variables and functions that need to be accessed across multiple files.

The following simple program shows the extern keyword in action:

1// globals.cpp
2int GlobalVar{42};

1// main.cpp
2#include <iostream>
3
4extern int GlobalVar;
5
6int main() {
7  std::cout << "GlobalVar: " << GlobalVar;
8}

1GlobalVar: 42

Explanation

• In globals.cpp, GlobalVar is defined.
• In main.cpp, extern int GlobalVar; declares that GlobalVar exists and will be defined elsewhere.
• This allows GlobalVar to be used in main.cpp without defining it there.

Here's an example with an extern function:

1// greeting.cpp
2#include <iostream>
3
4void SayHello() {
5  std::cout << "Hello from greeting\n";
6}

1// main.cpp
2#include <iostream>
3
4extern void SayHello();
5
6int main() {
7  SayHello();
8}

Explanation

• In greeting.cpp, SayHello() is defined.
• In main.cpp, extern void SayHello(); declares that SayHello() exists elsewhere.
• This allows SayHello() to be called in main.cpp.

Benefits of extern

1. Code Organization: It helps keep code modular and organized by allowing definitions to be separated from declarations.
2. Global Variables: Facilitates the use of global variables across different files without redefining them.

Common Mistakes

1. Multiple Definitions: Ensure variables defined with extern are only defined once across the entire program.
2. Matching Declarations: The extern declaration must match the definition in terms of type and qualifiers.

The extern keyword can be combined with other qualifiers, such as const:

1// math.cpp
2extern const float Pi{3.14159f};

1// main.cpp
2#include <iostream>
3
4extern const float Pi;
5
6int main() {
7  std::cout << "Pi: " << Pi;
8}

1Pi: 3.14159

In this case, Pi is a const variable with extern linkage. The extern keyword allows its use in main.cpp, even though it is defined in math.cpp.

The extern keyword is essential for managing linkage and ensuring variables and functions can be accessed across multiple files, promoting a modular and organized code structure.