Cpp Extern Keywords

C++ extern keywords

Created By: chatGPT

C++ extern is a storage class specifier that is used to declare a variable or function and indicate that it has external linkage. This means the variable or function can be accessed across different translation units. Using extern, we can share variables between multiple files. Notice that extern does not allocate memory for the variable; it merely provides a reference to it. Here’s an example:

 // File1.cpp
  int globalVar = 10; // Definition of globalVar

  // File2.cpp
  extern int globalVar; // Declaration of globalVar
  
  void printGlobalVar() {
      std::cout << "Global Variable: " << globalVar << std::endl;
  }

In the example above, we define globalVar in File1.cpp, and then declare it in File2.cpp using extern. This allows File2.cpp to access the globalVar defined in File1.cpp. It’s crucial to ensure that the actual definition is present in one of the files, or linking will fail due to an unresolved reference.

 // File1.cpp
  #include <iostream>

  int globalVar = 10; // Definition of globalVar

  int main() {
      std::cout << "From File1: " << globalVar << std::endl;
      return 0;
  }

  // File2.cpp
  #include <iostream>

  extern int globalVar; // Declaration of globalVar
  
  void printGlobalVar() {
      std::cout << "From File2: " << globalVar << std::endl;
  }

If we want to declare functions as extern, we use a similar approach. Declaring a function as extern allows it to be called from other files. Here’s an example:

 // FileA.cpp
  #include <iostream>

  extern void displayMessage(); // Declaration of the function

  int main() {
      displayMessage(); // Calling the extern function
      return 0;
  }

  // FileB.cpp
  #include <iostream>

  void displayMessage() {
      std::cout << "Hello from FileB!" << std::endl;
  }

In this example, displayMessage is declared in FileA.cpp using extern, and its definition exists in FileB.cpp. When FileA.cpp is compiled and linked with FileB.cpp, calling displayMessage() from FileA works seamlessly.

// FileA.cpp
#include <iostream>

extern void displayMessage(); // Declaration of function

int main() {
    displayMessage(); // Calling the extern function
    return 0;
}

// FileB.cpp
#include <iostream>

void displayMessage() {
    std::cout << "Hello from FileB!" << std::endl;
}