do classes go in .h files 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++

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

}