how to write an implementation file c++ code example

Example 1: what is a header in c++

// my_program.cpp
#include "my_class.h"

using namespace N;

int main()
{
    my_class mc;
    mc.do_something();
    return 0;
}

Example 2: what is a header in c++

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