# Connect multiple C++ Files {% file src="" %} Sample Code for linker using g++ {% endfile %} #### Here is a code example using multiple files:

// File: math_operations.h
#ifndef MATH_OPERATIONS_H
#define MATH_OPERATIONS_H

// Function declarations
int add(int a, int b);
int subtract(int a, int b);

// Variable declaration with external linkage
extern int globalValue;

#endif // MATH_OPERATIONS_H

// File: math_operations.cpp
#include "math_operations.h"

// Function definitions
int add(int a, int b) {
    return a + b;
}

int subtract(int a, int b) {
    return a - b;
}

// Variable definition
int globalValue = 10;

// File: main.cpp
#include <iostream>
#include "math_operations.h"

int main() {
    std::cout << "Sum: " << add(5, 3) << std::endl;
    std::cout << "Difference: " << subtract(10, 4) << std::endl;
    std::cout << "Global value: " << globalValue << std::endl;
    return 0;
}

*** Now, let's break down how these files are linked: 1. Header files and source files: * `math_operations.h` is a header file containing function declarations and an external variable declaration. * `math_operations.cpp` is a source file with the implementations of the functions declared in the header. * `main.cpp` is the main source file that uses the functions defined in `math_operations`. 2. \#include directive: * `math_operations.cpp` includes `math_operations.h` to ensure the function implementations match the declarations. * `main.cpp` includes `math_operations.h` to access the function declarations and the external variable. 3. External linkage: * `globalValue` is declared with `extern` in the header, making it accessible across multiple files. * It's defined in `math_operations.cpp` and can be used in `main.cpp`. 4. Function declarations and definitions: * Functions are declared in `math_operations.h` and defined in `math_operations.cpp`. * This allows `main.cpp` to use these functions by including only the header file. To compile and link these files, you would use a command like:

g++ main.cpp math_operations.cpp -o program

This compiles both source files and links them together into an executable named "program". The `#ifndef`, `#define`, and `#endif` in the header file create an "include guard" to prevent multiple inclusions of the same header, which can cause compilation errors.