using-declaration vs. using-directive

[Insert Pseudonym Here]

In FAQ 15.03, it is stated, "Note that a using directive does not declare
any variables; it merely makes names available." No similar mention is
made for using-declarations. Based on the name alone, I assume using-
declarations DO declare variables. Is this assumption correct? If so,
what's the practical difference between one of them declaring a variable,
and the other not? For that matter, what the theoretical difference?

 

[Rob Williscroft]

Insert Pseudonym Here wrote in
in comp.lang.c++:

In FAQ 15.03, it is stated, "Note that a using directive does not
declare any variables; it merely makes names available." No similar
mention is made for using-declarations. Based on the name alone, I
assume using- declarations DO declare variables. Is this assumption
correct?

Yes they declare names, but names that have been declared elseware.

If so, what's the practical difference between one of them
declaring a variable, and the other not? For that matter, what the
theoretical difference?

namespace A
{
int a;
};

namespace B
{
int a;

using namespace A; /* doesn't declare anything */
using A::a; /* whoops redeclares B::a */
};


Rob.