Title
Destructor lookup
Status
cd1
Section
11.4.7 [class.dtor]
Submitter
John Spicer

Created on 2000-09-06.00:00:00 last changed 161 months ago

Messages

Date: 2002-10-15.00:00:00

[Moved to DR at October 2002 meeting.]

Date: 2001-10-15.00:00:00

Proposed resolution (10/01):

Replace the normative text of 6.5.5 [basic.lookup.qual] paragraph 5 after the first sentence with:

Similarly, in a qualified-id of the form:

    ::opt nested-name-specifieropt class-name :: ~ class-name
the second class-name is looked up in the same scope as the first.

In 11.4.7 [class.dtor] paragraph 12, change the example to

D D_object;
typedef B B_alias;
B* B_ptr = &D_object;

void f() {
  D_object.B::~B();                //  calls  B's destructor
  B_ptr->~B();                    //  calls  D's destructor
  B_ptr->~B_alias();              //  calls  D's destructor
  B_ptr->B_alias::~B();           //  calls  B's destructor
  B_ptr->B_alias::~B_alias();     //  calls  B's destructor
}

April 2003: See issue 399.

Date: 2020-12-15.00:00:00

11.4.7 [class.dtor] contains this example:

    struct B {
        virtual ~B() { }
    };
    struct D : B {
        ~D() { }
    };

    D D_object;
    typedef B B_alias;
    B* B_ptr = &D_object;

    void f() {
        D_object.B::~B();               // calls B's destructor
        B_ptr->~B();                    // calls D's destructor
        B_ptr->~B_alias();              // calls D's destructor
        B_ptr->B_alias::~B();           // calls B's destructor
        B_ptr->B_alias::~B_alias();     // error, no B_alias in class B
    }

On the other hand, 6.5.5 [basic.lookup.qual] contains this example:

    struct C {
        typedef int I;
    };
    typedef int I1, I2;
    extern int* p;
    extern int* q;
    p->C::I::~I();       // I is looked up in the scope of C
    q->I1::~I2();        // I2 is looked up in the scope of
                         // the postfix-expression
    struct A {
        ~A();
    };
    typedef A AB;
    int main()
    {
        AB *p;
        p->AB::~AB();    // explicitly calls the destructor for A
    }

Note that

     B_ptr->B_alias::~B_alias();

is claimed to be an error, while the equivalent

     p->AB::~AB();

is claimed to be well-formed.

I believe that clause 3 is correct and that clause 12 is in error. We worked hard to get the destructor lookup rules in clause 3 to be right, and I think we failed to notice that a change was also needed in clause 12.

Mike Miller:

Unfortunately, I don't believe 6.5.5 [basic.lookup.qual] covers the case of p->AB::~AB(). It's clearly intended to do so, as evidenced by 6.5.5.2 [class.qual] paragraph 1 ("a destructor name is looked up as specified in 6.5.5 [basic.lookup.qual]"), but I don't think the language there does so.

The relevant paragraph is 6.5.5 [basic.lookup.qual] paragraph 5. (None of the other paragraphs in that section deal with this topic at all.) It has two parts. The first is

If a pseudo-destructor-name (_N4778_.7.6.1.4 [expr.pseudo]) contains a nested-name-specifier, the type-names are looked up as types in the scope designated by the nested-name-specifier.

This sentence doesn't apply, because ~AB isn't a pseudo-destructor-name. _N4778_.7.6.1.4 [expr.pseudo] makes clear that this syntactic production (7.6.1 [expr.post] paragraph 1) only applies to cases where the type-name is not a class-name. p->AB::~AB is covered by the production using id-expression.

The second part of 6.5.5 [basic.lookup.qual] paragraph 5 says

In a qualified-id of the form:

    ::opt nested-name-specifier ~ class-name

where the nested-name-specifier designates a namespace name, and in a qualified-id of the form:

    ::opt nested-name-specifier class-name :: ~ class-name

the class-names are looked up as types in the scope designated by the nested-name-specifier.

This wording doesn't apply, either. The first one doesn't because the nested-name-specifier is a class-name, not a namespace name. The second doesn't because there's only one layer of qualification.

As far as I can tell, there's no normative text that specifies how the ~AB is looked up in p->AB::~AB(). 6.5.5.2 [class.qual], where all the other class member qualified lookups are handled, defers to 6.5.5 [basic.lookup.qual], and 6.5.5 [basic.lookup.qual] doesn't cover the case.

See also issue 305.

Jason Merrill: My thoughts on the subject were that the name we use in a destructor call is really meaningless; as soon as we see the ~ we know what the user means, all we're doing from that point is testing their ability to name the destructor in a conformant way. I think that everyone will agree that

  anything::B::~B()
should be well-formed, regardless of the origins of the name "B". I believe that the rule about looking up the second "B" in the same context as the first was intended to provide this behavior, but to me this seems much more heavyweight than necessary. We don't need a whole new type of lookup to be able to use the same name before and after the ~; we can just say that if the two names match, the call is well-formed. This is significantly simpler to express, both in the standard and in an implementation.

Anyone writing two different names here is either deliberately writing obfuscated code, trying to call the destructor of a nested class, or fighting an ornery compiler (i.e. one that still wants to see B_alias::~B()). I think we can ignore the first case. The third would be handled by reverting to the old rule (look up the name after ~ in the normal way) with the lexical matching exception described above -- or we could decide to break such code, do no lookup at all, and only accept a matching name. In a good implementation, the second should probably get an error message telling them to write Outer::Inner::~Inner instead.

We discussed this at the meetings, but I don't remember if we came to any sort of consensus on a direction. I see three options:

  1. Stick with the status quo, i.e. the special lookup rule such that if the name before ::~ is a class name, the name after ::~ is looked up in the same scope as the previous one. If we choose this option, we just need better wording that actually expresses this, as suggested in the issue list. This option breaks old B_alias::~B code where B_alias is declared in a different scope from B.
  2. Revert to the old rules, whereby the name after ::~ is looked up just like a name after ::, with the exception that if it matches the name before ::~ then it is considered to name the same class. This option supports old code and code that writes B_alias::~B_alias. It does not support the q->I1::~I2 usage of 6.5.5 [basic.lookup.qual], but that seems like deliberate obfuscation. This option is simpler to implement than #1.
  3. Do no lookup for a name after ::~; it must match the name before. This breaks old code as #1, but supports the most important case where the names match. This option may be slightly simpler to implement than #2. It is certainly easier to teach.

My order of preference is 2, 3, 1.

Incidentally, it seems to me oddly inconsistent to allow Namespace::~Class, but not Outer::~Inner. Prohibiting the latter makes sense from the standpoint of avoiding ambiguity, but what was the rationale for allowing the former?

John Spicer: I agree that allowing Namespace::~Class is odd. I'm not sure where this came from. If we eliminated that special case, then I believe the #1 rule would just be that in A::B1::~B2 you look up B1 and B2 in the same place in all cases.

I don't like #2. I don't think the "old" rules represent a deliberate design choice, just an error in the way the lookup was described. The usage that rule permits p->X::~Y (where Y is a typedef to X defined in X), but I doubt people really do that. In other words, I think that #1 a more useful special case than #2 does, not that I think either special case is very important.

One problem with the name matching rule is handling cases like:

  A<int> *aip;

  aip->A<int>::~A<int>();  // should work
  aip->A<int>::~A<char>(); // should not
I would favor #1, while eliminating the special case of Namespace::~Class.

History
Date User Action Args
2008-10-05 00:00:00adminsetstatus: wp -> cd1
2003-04-25 00:00:00adminsetstatus: dr -> wp
2002-11-08 00:00:00adminsetmessages: + msg778
2002-11-08 00:00:00adminsetstatus: ready -> dr
2002-05-10 00:00:00adminsetstatus: review -> ready
2002-03-11 00:00:00adminsetmessages: + msg584
2002-03-11 00:00:00adminsetstatus: drafting -> review
2001-11-09 00:00:00adminsetstatus: open -> drafting
2000-09-06 00:00:00admincreate