Build error with header file

Question

I cannot create classes with header file. When I create the file . h and create the implementation of member functions in the file . cpp gives error when compiling. I thought it was me, so I went to the site Microsoft an example they gave. It was an error in my compiler, but in an online compiler it worked.

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

using namespace N;

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

// 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;
}

// my_class.h
namespace N
{
    class my_class
    {
    public:
        void do_something();
    };

}