Why can't namespaces be template parameters?

问题

I understand that namespaces cannot be template parameters. See the question, "template specialized on a namespace":

Given:

namespace A {
  class Foo;
  class Bar;
}

namespace B {
  class Foo;
  class Bar;
}

I want to template a class on the namespace A or B such that the following works:

template<name> class C {
  name::Foo* foo;
  name::Bar* bar;
};

I was wondering why this is the case. I understand that templates aren't structures, but is there a technical limitation to the compiler's design? Or is there some significant trade off for implementing this functionality?

回答1:

This would be:

1. (IMO) Inappropriate: Namespaces avoid name clashes. Polymorphism is outside their charter.
2. Unnecessary: It would achieve nothing that can't already be done with structs.
3. Possibly difficult: A namespace isn't a complete, self-contained entity. Different members of a namespace can be declared in different headers and even different compilation units.

回答2:

Back when Bjarne Stroustrup first started talking about templates in C++ standards meetings he mentioned namespaces as template parameters. The reaction was skeptical, in part because namespaces themselves were so new, and we were afraid of combining two things that we didn't understand.

来源：https://stackoverflow.com/questions/12905951/why-cant-namespaces-be-template-parameters