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 name

Answer 1

Okay, you say that std is only an example. But to me it appears your question is only applicable to a case where somebody does something as evil as overriding std.

Let me provide another example:

namespace logging
{
    // Logs to local file
}

namespace server
{
    namespace logging
    {
        // Logs to remote destination
    }
}

namespace client
{
    namespace logging
    {
        // Logs to local file and to screen
    }
}

In this case, not prepending :: results in default behavior.

Answer 2

This is so much artificial:

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

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

} // namespace std
} // namespace fred

This Fred would not be able to explain why he did that to the whole team.

To answer your question: people omit first "::" because it is a habit, which saves time and because it is easier to expect that when namespace is used anywhere, then such name is absolute, not relative.