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c++language-lawyerc++23

What is the meaning of Note 1 in the C++ class member name lookup rules?

From http://eel.is/c++draft/class.member.lookup#1 :

A search in a scope X for a name N from a program point P is a single search in X for N from P unless X is the scope of a class or class template T, in which case the following steps define the result of the search.

[Note 1: The result differs only if N is a conversion-function-id or if the single search would find nothing. — end note]

I'm having a hard time making a sense of the Note. It seems that a "single search" from a class scope will find preceding declarations at namespace scope, since the namespace scope contains the class scope. But, as we know, if the name has also been declared as a member of a non-dependent base class, then the base class member takes precedence over the namespace member. Note 1 seems to contradict this, since it's basically saying "if N is not a conversion-function-id, then you can just do a normal single search, and only if you fail to find anything, then use the procedure in this section". But the single search will succeed by finding the namespace scope declaration, and the class member lookup will yield a different result.

Where is the error in my understanding?

Solution

  • Answer

    A single search considers only one scope—not an enclosing namespace or even a base class. It’s an unqualified search that considers all enclosing scopes. Single searches and (plain) searches are subroutines of these higher-level procedures.

    Context

    It should be said, since there have been a lot of these questions lately, that these terms exist to reduce ambiguity and imprecision (e.g., CWG issue 191) in the definitions of “programmer-level” constructs like (un)qualified name lookup. I didn’t invent them to increase the number of vocabulary words that the typical programmer should be expected to have memorized. (Put differently, the standard is not a tutorial.)

    Of course, there’s nothing special about this particular question in this regard, but I must hope that this will thereby tend to find the people that need to see it.