c++ - Why does everybody use unanchored namespace declarations (i.e. std:: not ::std::)?

Question

It seems to me that using unanchored namespaces is just asking for trouble later when someone puts in a new namespace that happens to have the same name as a root level namespace and mysteriously alters the meaning of a whole lot of programs. So, why do people always say std:: instead of ::std::. Do they really mean to be saying "I want to use whatever std is handy, not the root one."?

Here is an example of what I mean:

In fred/Foo.h:

#include <string>

namespace fred {

class Foo {
 public:
   void aPublicMember(const std::string &s);
};

} // end namespace fred

In fred/Bar.h:

namespace fred {
namespace std {  // A standard fred component

class string { // Something rather unlike the ::std::string
   // ...
};

} // namespace std

class Bar {
 public:
   void aPublicMember(std::string &s);
};

} // namespace fred

In oops.cpp:

#include <string>
#include "fred/Bar.h"
#include "fred/Foo.h"  // Oops, the meaning of Foo is now different.

Is that what people want, or am I missing something?

And maybe you say that you should just never name a namespace std. And that's all well and good, but what about some other root level namespace then? Should any root level namespace anybody ever defines anywhere always be off-limits for a sub-namespace name?

To clarify, I won't consider any answer that tells me std is special because I just used it as an example. I'm talking about a general issue, and I'm using std as a prop to illustrate it, though I do admit it's a rather startling prop.

Answer 1

So, why do people always say std:: instead of ::std::

Probably because they never really had problems with ambiguity because of it.

Along the same lines: I never had to include "earth" in my address, and I'm not going to.

You have to draw the line somewhere, and this it is a reasonable assumption that others won't make their own std namespaces, or at least that a library that does won't be very popular. :)