This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-12-19
9.3.4 [dcl.meaning] paragraph 1 says:
In the qualified declarator-id for a class or namespace member definition that appears outside of the member's class or namespace, the nested-name-specifier shall not name any of the namespaces that enclose the member's definition.This results in the following behavior:
namespace N {
namespace M {
void f();
void g();
}
void M::f(){} // okay
void N::M::g(){} // error
}
I was very surprised when this rule was pointed out to me. The change
appears to have been introduced around the time of the first Santa
Cruz meeting, but I don't recall discussion of it and could not find a
motion related to it.
Regardless of where it came from, I also can't understand why it is there. Certainly it shouldn't matter how you name a given class or namespace.
For example, the standard permits:
namespace N {
namespace M {
void f();
void g();
}
namespace X = M;
namespace Y = N::M;
void X::f(){} // okay
void Y::g(){} // okay
}
So, it is okay to use an alias for N::M,
but not to use N::M directly.
Note that it is okay to use N::M
in any other context at this point
in the program (i.e., the rule is a specific restriction on declarator
names, not a general rule on the use of qualified names).
Does anyone recall the intent of this rule or any rationale for its existence?
Notes from 04/00 meeting:
There was some question as to whether this issue actually constituted a defect in the Standard. John Spicer suggested that machine-generated source code would be likely to run afoul of this prohibition. Francis Glassborow expressed support for a rule that would allow full qualification, or qualification relative to the namespace containing the definition, but not qualification relative to a containing namespace. There was no consensus for moving forward with a DR at this point, so the issue was left in "review" status.
Proposed resolution (10/00):
Remove the last sentence of 9.3.4 [dcl.meaning] paragraph 1 (cited above) and the example that follows.