问题
It's possible to do using namespace foo::bar; (i.e., using the inner namespace without using the outer namespace first / at all), why does the standard forbid to do the following?
namespace foo::bar {
// open nested namespace bar in foo and extend it...
}
I'm not looking for a workaround, just a possible rational on why this isn't allowed.
回答1:
I'm not sure "forbidden" is the right word - maybe it was just an oversight. It's a fairly small nice-to-have which isn't really a big deal.
You could also take the point of view that the namespace foo isn't created yet when you write foo::bar, so allowing that syntax makes it look like foo was already created when it was not.
You could also go further and request the ability to write class Foo::MyClass {... to define MyClass in namespace Foo, and the same for functions, variables, etc. But is this feature really necessary and solving any particular pressing problem?
回答2:
Why make compilers implement it when you could just do
namespace foo { namespace bar {
}}
来源：https://stackoverflow.com/questions/6322685/why-is-it-forbidden-to-open-multiple-namespaces-at-a-stretch