what to define in a .h file c++ code example

Example 1: defining class in other file in c++

//ClassOne.hpp:

class ClassOne
{
public:
  ClassOne(); // note, no function body        
  int method(); // no body here either
private:
  int member;
};
//ClassOne.cpp:

#include "ClassOne.hpp"

// implementation of constructor
ClassOne::ClassOne()
 :member(0)
{}

// implementation of "method"
int ClassOne::method()
{
  return member++;
}
//main.cpp:

#include "ClassOne.hpp" // Bring the ClassOne declaration into "view" of the compiler

int main(int argc, char* argv[])
{
  ClassOne c1;
  c1.method();

  return 0;
}

Example 2: 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;
}