c++ how to use functions from .cpp file in .h file code example

Example 1: 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;
}

Example 2: what is a header in c++

int x; // declaration
x = 42; // use x