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

The practical reason for unanchored namespaces is that one level of namespaces usually is enough. When it isn't, a second level is usually going to be used for implementation details. And finally, even when using multiple levels, they are still usually specified implicitly from root level. ie. even inside namespace ns1, you'd typically refer to ns1::ns2::foo instead of ns2::foo or ::ns1::ns2::foo.

So, for these three reasons the ::ns1 form is redundant in normal cases. The only case where I'd consider it would be in submissions to Boost, because as a Boost author I won't know where my software will be used.