How to Create a Header File in C++

1. Use .h or .hpp Suffixes to Create a Header File in C++
2. Use Header Files to Break Down Separate Function Blocks of the Program in Modules

This article will explain several methods of how to create a header file in C++.

Use .h or .hpp Suffixes to Create a Header File in C++

Contemporary programs are rarely written without libraries, which are code constructs implemented by others. C++ provides special token - #include to import needed library header files and external functions or data structures. Note that, generally, library header files have a particular filename suffix like library_name.h or library_name.hpp. C++ program structure provides the concept of header files to ease the usage of certain reusable code blocks. Thus, users can create their own header files and include them in source files as needed.

Suppose that the user needs to implement a class named Point containing two data members of type double. The class has two constructors and + operator defined in it. It also has a print function to output both data members’ values to the cout stream. Generally, there are also header guards enclosing the Point class definition to ensure no name clashes occur when including in relatively large programs. Note that there should be a consistent naming scheme for the variables names defined after include guard; usually, these variables are named after the class itself.

#ifndef POINT_H
#define POINT_H

class Point {
  double x, y;

 public:
  Point();
  Point(double, double);
  Point operator+(const Point &other) const;
  void print();
};

#endif

Another method to structure a header file for the Point class is to include the function implementation code in the same file. Note that putting the previous code snippet into a Point.hpp file and including it will raise several undefined errors. Since the functions are defined in the following example code, we can include it as a Point.hpp header file and use the class with its methods.

#include <iostream>
#include <vector>

#ifndef POINT_H
#define POINT_H

class Point {
  double x, y;

 public:
  Point();
  Point(double, double);
  Point operator+(const Point &other) const;
  void print();
};

Point::Point() { x = y = 0.0; }

Point::Point(double a, double b) {
  x = a;
  y = b;
}

Point Point::operator+(const Point &other) const {
  return {x + other.x, y + other.y};
}

void Point::print() { std::cout << "(" << x << "," << y << ")" << std::endl; }

#endif

Use Header Files to Break Down Separate Function Blocks of the Program in Modules

Alternatively, one can utilize a module-based separation scheme for headers and corresponding source files of the given class to implement a more flexible project file structure. In this design, one should define each functionally distinct class in a separate .hpp header file and implement its methods in a source file with the same name. Once the class’s needed header file is included in the main source file and compiled, the preprocessor will combine the code blocks from all included header files, and the result would be the same as compiling the following source code, which implements the functionality in a single source file.

#include <iostream>
#include <vector>

#ifndef POINT_H
#define POINT_H

class Point {
  double x, y;

 public:
  Point();
  Point(double, double);
  Point operator+(const Point &other) const;
  void print();
};

Point::Point() { x = y = 0.0; }

Point::Point(double a, double b) {
  x = a;
  y = b;
}

Point Point::operator+(const Point &other) const {
  return {x + other.x, y + other.y};
}

void Point::print() { std::cout << "(" << x << "," << y << ")" << std::endl; }
#endif

using std::cin;
using std::cout;

int main() {
  double x, y;

  cin >> x >> y;
  Point a1(x, y);
  cin >> x >> y;
  Point a2(x, y);

  cout << "a1: ";
  a1.print();
  cout << "a2: ";
  a2.print();

  a1 = a1 + a2;
  cout << "a1+a2: ";
  a1.print();

  exit(EXIT_SUCCESS);
}