Where should "using" be placed

[JayRyGeo]

I am writing a program that has one header file. However, I have always put "using namespace std;" after all the includes which means that the header file never sees that declaration. The problem that I have come across is when I try to set a function to return a string, I would have to put std::string in order for the program to compile.

So my question is should I use std::string in the header, or should I include "using namespace std" inside the header file?

source.cpp

 #include <iostream>
#include <string>
#include "functions.h"
using namespace std;

int main()
{
    
}

functions.h

#ifndef FUNCTIONS_H
#define FUNCTIONS_H

class Student
{

  public:

      int getID();
      string getFullName();
      string getDOB();
      int getYearStarted();
      string getAddress();
      void changeName( string );
      void changeDOB( int, int, int );
      void changeYearStarted( int );
      void changeAddress();
      string toString();

  private:

};

#endif 

[Chervil]

It's bad practice to put using namespace std; in a header file, as any file which includes the header will find itself with that using whether it wants it or not.

Better to explicitly put std::string in the header.

[JayRyGeo]

Thank you, I was confused about this convention and I'm trying to stick to them as much as possible so I'm not confused in the future.