Beautiful C++: 30 Core Guidelines for Writing Clean, Safe, and Fast Code

Chapter 3.6

SF.7: Don’t write using namespace at global scope in a header file

Don’t do this

Please, don’t do this. Ever.

Never. Not ever. Not even once.

Please.

The Core Guidelines document offers the following example:

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// bad.h
#include <iostream>
using namespace std; // bad <- "You're not kidding," says Guy

// user.cpp
#include "bad.h"

// some function that happens to be named copy
bool copy(/*... some parameters ...*/);

int main()
{
  // now overloads local ::copy and std::copy, could be ambiguous
  copy(/*...*/);
}

“Bad” is an understatement. We know it seems we are belaboring the point, but this example only exposes part of the horror.

Disambiguation

There is one and only one global namespace. From the standard, ...