* cp-tree.h (DECL_NAMESPACE_ASSOCIATIONS): New macro.

        * name-lookup.c (parse_using_directive): New fn.
        (is_associated_namespace): New fn.
        (arg_assoc_namespace): Also check associated namespaces.
        * name-lookup.h: Declare new fns.
        * pt.c (maybe_process_partial_specialization): Allow
        specialization in associated namespace.
        * parser.c (cp_parser_using_directive): Accept attributes.  Use
        parse_using_directive.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@73468 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 40 insertions, 0 deletions

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index fdc1c7c6827..7825a12376d 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -7649,6 +7649,7 @@ Predefined Macros,cpp,The GNU C Preprocessor}).
 * Bound member functions:: You can extract a function pointer to the
                         method denoted by a @samp{->*} or @samp{.*} expression.
 * C++ Attributes::      Variable, function, and type attributes for C++ only.
+* Strong Using::      Strong using-directives for namespace composition.
 * Java Exceptions::     Tweaking exception handling to work with Java.
 * Deprecated Features:: Things will disappear from g++.
 * Backwards Compatibility:: Compatibilities with earlier definitions of C++.
@@ -8249,6 +8250,45 @@ interface table mechanism, instead of regular virtual table dispatch.
 
 @end table
 
+See also @xref{Strong Using}.
+
+@node Strong Using
+@section Strong Using
+
+A using-directive with @code{__attribute ((strong))} is stronger
+than a normal using-directive in two ways:
+
+@itemize @bullet
+@item
+Templates from the used namespace can be specialized as though they were members of the using namespace.
+
+@item
+The using namespace is considered an associated namespace of all
+templates in the used namespace for purposes of argument-dependent
+name lookup.
+@end itemize
+
+This is useful for composing a namespace transparently from
+implementation namespaces.  For example:
+
+@smallexample
+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>());
+@}
+@end smallexample
+
 @node Java Exceptions
 @section Java Exceptions