how to use a header file in c++ code example

Example 1: c++ include header

#include "headerfile.h"

Example 2: what is a header in c++

// my_class.cpp
#include "my_class.h" // header in local directory
#include <iostream> // header in standard library

using namespace N;
using namespace std;

void my_class::do_something()
{
    cout << "Doing something!" << endl;
}