#include <iostream>    //no .h

int main(void) {
	cout << "Hello." << endl;
	return 0;
}

// compile errors:
// 'cout' : undeclared identifier
// '<<' : illegal, right operand has type 'char [7]'
// 'endl' : undeclared identifier

#include <cstdio>    // no .h
#include <iostream.h>    // .h

// Let this file be our header with data-types stored in it.

namespace myTypes
{
  enum RPS { ROCK, PAPER, SCISSORS };
  struct player
  {
    char *name;
    RPS choice;
  };
}
namespace myOtherTypes
{
  enum Weather { RAIN, SHINE, WIND, HAIL, SNOW };

  namespace nestedTypes
  {
    enum TimeZone { PST, EST, MNT };
  }
}

// Let this be an implementation file #1
#include "thatheaderfile.h"

using namespace myTypes; // bring myTypes into scope

int main()
{
  player newGuy; // in scope because of the using namespace
  RPS    lame_variable; // also in scope because using namespace
  
  return 0;
}

// Let this be an implementation file #2
#include "thatheaderfile.h"

using myTypes::player; // brings only the "player" type into scope, nothing else

int main()
{
  player newGuy; // in scope because of scope operator
  RPS    lame_variable; // error: we don't see RPS; we only see "player".
  
  return 0;
}

// This isn't an implementation file... just some examples

using namespace myOtherTypes; // Brings everything in this namespace into scope, even namespaces nested within it(?)
using namespace myOtherTypes::nestedTypes; // Brings only nestedTypes into scope
using myOtherTypes::nestedTypes::TimeZone; // Brings only TimeZone type into scope