Argument-dependent lookup and operator functions
6.5.4 [basic.lookup.argdep]
Herb Sutter

Created on 2002-09-18.00:00:00 last changed 232 months ago


Date: 2003-04-15.00:00:00

Notes from the April 2003 meeting:

The ADL rules in the standard sort of look at if they are fully recursive, but in fact they are not; in some cases, enclosing classes and base classes are considered, and in others they are not. Microsoft and g++ did fully-recursive implementations, and EDG and IBM did it the other way. Jon Caves reports that Microsoft saw no noticeable difference (e.g., no complaints from customers internal or external) when they made this change, so we believe that even if the rules are imperfect the way they are in the standard, they are clear and the imperfections are small enough that programmers will not notice them. Given that, it seemed prudent to make no changes and just close this issue.

The template-id issue is spun off as issue 403.

Date: 2004-09-10.00:00:00

I believe the following code example should unambiguously call the member operator+. Am I right?

  //--- some library header ---
  namespace N1 {
    template<class T> struct Base { };

    template<class T> struct X {
      struct Y : public Base<T> {     // here's a member operator+
        Y operator+( int _Off ) const { return Y(); }

      Y f( unsigned i ) { return Y() + i; } // the "+" in question

  //--- some user code ---
  namespace N2 {
    struct Z { };

    template<typename T>              // here's another operator+
    int* operator+( T , unsigned ) { static int i ; return &i ; }

  int main() {
    N1::X< N2::Z > v;
    v.f( 0 );

My expectation is that 6.5.4 [basic.lookup.argdep] would govern, specifically:

If the ordinary unqualified lookup of the name finds the declaration of a class member function, the associated namespaces and classes are not considered.
So I think the member should hide the otherwise-better-matching one in the associated namespace. Here's what compilers do:

Agree with me and call the member operator+: Borland 5.5, Comeau, EDG 3.0.1, Metrowerks 8.0, MSVC 6.0

Disagree with me and try to call N2::operator+: gcc 2.95.3, 3.1.1, and 3.2; MSVC 7.0

Simple so far, but someone tells me that [over.match.oper] muddies the waters. There, paragraph 10 summarizes that subclause:

[Note: the lookup rules for operators in expressions are different than the lookup rules for operator function names in a function call, ...
In particular, consider the above call to "Y() + unsigned" and please help me step through [over.match.oper] paragraph 3:
... for a binary operator @ with a left operand of a type whose cv-unqualified version is T1 and a right operand of a type whose cv-unqualified version is T2,
OK so far, here @ is +, and T1 is N1::X::Y.
three sets of candidate functions, designated member candidates, non-member candidates and built-in candidates, are constructed as follows:
[and later are union'd together to get the candidate list]
If T1 is a class type, the set of member candidates is the result of the qualified lookup of T1::operator@ (over.call.func); otherwise, the set of member candidates is empty.
So there is one member candidate, N1::X::Y::operator+.
The set of non-member candidates is the result of the unqualified lookup of operator@ in the context of the expression according to the usual rules for name lookup in unqualified function calls (basic.lookup.argdep) except that all member functions are ignored.

*** This is the question: What does that last phrase mean? Does it mean:

a) first apply the usual ADL rules to generate a candidate list, then ignore any member functions in that list (this is what I believe and hope it means, and in particular it means that the presence of a member will suppress names that ADL would otherwise find in the associated namespaces); or

b) something else?

In short, does N2::operator+ make it into the candidate list? I think it shouldn't. Am I right?

John Spicer: I believe that the answer is sort-of "a" above. More specifically, the unqualified lookup consists of a "normal" unqualified lookup and ADL. ADL always deals with only namespace members, so the "ignore members functions" part must affect the normal lookup, which should ignore class members when searching for an operator.

I suspect that the difference between compilers may have to do with details of argument-dependent lookup. In the example given, the argument types are "N1::X<N2::Z>::Y" and "unsigned int". In order for N2::operator+ to be a candidate, N2 must be an associated namespace.

N1::X<N2::Z>::Y is a class type, so 6.5.4 [basic.lookup.argdep] says that its associated classes are its direct and indirect base classes, and its namespaces are the namespaces of those classes. So, its associated namespace is just N1.

6.5.4 [basic.lookup.argdep] also says:

If T is a template-id, its associated namespaces and classes are the namespace in which the template is defined; for member templates, the member template's class; the namespaces and classes associated with the types of the template arguments provided for template type parameters (excluding template template parameters); the namespaces in which any template template arguments are defined; and the classes in which any member templates used as template template arguments are defined. [Note: non-type template arguments do not contribute to the set of associated namespaces. ]
First of all, there is a problem with the term "is a template-id". template-id is a syntactic constuct and you can't really talk about a type being a template-id. Presumably, this is intended to mean "If T is the type of a class template specialization ...". But does this apply to N1::X<N2::Z>::Y? Y is a class nested within a class template specialization. In addition, its base class is a class template specialization.

I think this raises two issues:

  1. Should the enclosing class(es) of a class, and their template argument lists (if any) contribute to the associated classes/namespaces for ADL?
  2. Should the template argument lists of base classes contribute to the associated classes/namespaces for ADL?
Date User Action Args
2003-04-25 00:00:00adminsetmessages: + msg878
2003-04-25 00:00:00adminsetstatus: open -> nad
2002-09-18 00:00:00admincreate