overloading << and using namespace std

Question

I need a help with understanding the following:

I overloaded << operator. I wrote a test program. I didnt include the "using namespace std" code. But the program worked out well.

#include <iostream>
#include "fraction.h"

//using namespace std;

int main(void)
{
//constructing two fractions
    Fraction a(4, 2);
    Fraction b(17, 11);
//modifying them that is entering a fractions from keyboard
    cin>>a;
    cin>>b;
//computing product and quotient and printing them using cout
    cout<<a*b<<endl;
    cout<<a/b<<endl;
}

But as you can see I used "endl" which is from standard namespace. Can you explain me the "paradox" I m getting here.

PS I didnt include .h and .cpp file because I think they are irrelevant.

Answer 1

A compiler works with translation unit. It takes a .cpp and then insert .h text inside the translation unit. If your header file includes using namespace std; , it will effectively be present in the code the compiler tries to compile.

This is a bad practice as your are shoving namespace using down your user throat and can induce name clashes if other namespace use the same name.

Answer 2

If you can use cin and cout , and the problem is using endl , then your fraction.h file is most likely including those dependencies either in one of two ways

using std::cin;
using std::cout;
using std::endl;

or

using namespace std;

Answer 3

When you #include something, is does a straight text replacement with the file. Since you #include "fraction.h" and it probably has the line

using namespace std;

the main file also uses std .

Also note, it is not good practice to using namespace std; in header files since everywhere else that includes the header will also automatically use std which can cause conflicts if the programmer is unaware.