using namespace std; in a header file

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Let’s say I declare a class string myself. Because I’m a lazy bum, I do so in global namespace.

// Solar's stuff
class string
{
    public:
        string();
        // ...
};

Some time later on, I realize that re-using some of your code would benefit my project. Thanks to you making it Open Source, I can do so:

#include <solarstuff.hpp>
#include <phoenixstuff.hpp>

string foo;

But suddenly the compiler doesn’t like me anymore. Because there is a ::string (my class) and another ::string (the standard one, included by your header and brought into global namespace with using namespace std;), there’s all kinds of pain to be had.

Worse, this problem gets promoted through every file that includes my header (which includes your header, which… you get the idea.)

Yes I know, in this example I am also to blame for not protecting my own classes in my own namespace, but that’s the one I came up with ad-hoc.

Namespaces are there to avoid clashes of identifiers. Your header not only introduces MyStuff into the global namespace, but also every identifier from string and fstream. Chances are most of them are never actually needed by either of us, so why dragging them into global, polluting the environment?

Addition: From the view of a maintenance coder / debugger, foo::MyStuff is ten times more convenient than MyStuff, namespace’d somewhere else (probably not even the same source file), because you get the namespace information right there at the point in the code where you need it.

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