Header Files & Source Files

Part of the Object Oriented Programming section of Coddy's C++ journey — lesson 3 of 104.

In C++, code is split into header files (.h) for declarations and source files (.cpp) for implementations.

Header file with header guards (helpers.h)

#ifndef HELPERS_H
#define HELPERS_H

#include <string>

std::string greet(std::string name);
int addNumbers(int a, int b);

#endif

Source file with implementations (helpers.cpp)

#include "helpers.h"

std::string greet(std::string name) {
    return "Hello, " + name + "!";
}

int addNumbers(int a, int b) {
    return a + b;
}

Using them in main

#include <iostream>
#include "helpers.h"

int main() {
    std::cout << greet("Alice") << std::endl;
    std::cout << addNumbers(5, 3) << std::endl;
    return 0;
}

Output:

Hello, Alice!
8

Header guards (#ifndef, #define, #endif) prevent a header from being included multiple times. The .h file declares what exists (prototypes), the .cpp file defines how it works (implementations).

Challenge

Medium

Create three helper functions split across a header file and source file:

helpers.h — Function declarations with header guards
helpers.cpp — Function implementations

Functions to create:

greet(name) — returns "Hello, <name>!"
ageInTenYears(age) — returns age + 10
checkAdult(name, age) — returns "<name> is an adult" if age >= 18, otherwise "<name> is a minor"

Cheat sheet

C++ code is organized into header files (.h) for declarations and source files (.cpp) for implementations.

Header guards prevent multiple inclusions:

#ifndef HELPERS_H
#define HELPERS_H
// declarations here
#endif

Header file (helpers.h) contains function declarations:

#ifndef HELPERS_H
#define HELPERS_H

#include <string>

std::string greet(std::string name);
int addNumbers(int a, int b);

#endif

Source file (helpers.cpp) contains implementations:

#include "helpers.h"

std::string greet(std::string name) {
    return "Hello, " + name + "!";
}

int addNumbers(int a, int b) {
    return a + b;
}

Using the functions in main:

#include <iostream>
#include "helpers.h"

int main() {
    std::cout << greet("Alice") << std::endl;
    std::cout << addNumbers(5, 3) << std::endl;
    return 0;
}

Try it yourself

#include <iostream>
#include "helpers.h"

int main() {
    std::string name;
    int age;
    std::getline(std::cin, name);
    std::cin >> age;
    
    std::cout << greet(name) << std::endl;
    std::cout << "In 10 years you will be " << ageInTenYears(age) << std::endl;
    std::cout << checkAdult(name, age) << std::endl;
    return 0;
}