6.9 Strong Using
Caution: The semantics of this extension are not fully
defined. Users should refrain from using this extension as its
semantics may change subtly over time. It is possible that this
extension wil be removed in future versions of G++.
A using-directive with __attribute ((strong)) is stronger
than a normal using-directive in two ways:
- Templates from the used namespace can be specialized as though they were members of the using namespace.
- The using namespace is considered an associated namespace of all
templates in the used namespace for purposes of argument-dependent
name lookup.
This is useful for composing a namespace transparently from
implementation namespaces. For example:
namespace std {
namespace debug {
template <class T> struct A { };
}
using namespace debug __attribute ((__strong__));
template <> struct A<int> { }; // ok to specialize
template <class T> void f (A<T>);
}
int main()
{
f (std::A<float>()); // lookup finds std::f
f (std::A<int>());
}
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