[Voted into the WP at the June, 2008 meeting.]

Proposed resolution (February, 2008):

1. Change 7.6.1.3 [expr.call] paragraph 1 as follows:

   ... For a member function call, the postfix expression shall be an implicit (11.4.3 [class.mfct.non.static], 11.4.9 [class.static]) or explicit class member access (7.6.1.5 [expr.ref]) whose id-expression is a function member name, or a pointer-to-member expression (7.6.4 [expr.mptr.oper]) selecting a function member. The first expression in the postfix expression is then called the object expression, and; the call is as a member of the object pointed to or referred to by the object expression (7.6.1.5 [expr.ref], 7.6.4 [expr.mptr.oper]). In the case of an implicit class member access, the implied object is the one pointed to by this. [Note: a member function call of the form f() is interpreted as (*this).f() (see 11.4.3 [class.mfct.non.static]). —end note] If a function or member function name is used, the name can be overloaded ( Clause 12 [over]), in which case the appropriate function shall be selected according to the rules in 12.2 [over.match]. The function called in a member function call is normally selected according to the static type of the object expression (11.7 [class.derived]), but if that function is virtual and is not specified using a qualified-id then the function actually called will be the final overrider (11.7.3 [class.virtual]) of the selected function in the dynamic type of the object expression If the selected function is non-virtual, or if the id-expression in the class member access expression is a qualified-id, that function is called. Otherwise, its final overrider (11.7.3 [class.virtual]) in the dynamic type of the object expression is called. ...

2. Change 7.6.4 [expr.mptr.oper] paragraph 4 as follows:

   The first operand is called the object expression. If the dynamic type of the object expression does not contain the member to which the pointer refers, the behavior is undefined.

Notes from October 2002 meeting:

This was moved back to open for lack of a champion. Martin O'Riordan is not expected to be attending meetings.

Martin O'Riordan: Having gone through all the relevant references in the IS, it is not conclusive that a call via a pointer to a virtual member function is polymorphic at all, and could legitimately be interpreted as being static.

Consider 7.6.1.3 [expr.call] paragraph 1:

The function called in a member function call is normally selected according to the static type of the object expression ( 11.7 [class.derived] ), but if that function is virtual and is not specified using a qualified-id then the function actually called will be the final overrider (11.7.3 [class.virtual] ) of the selected function in the dynamic type of the object expression.

Yet other references such as 7.6.4 [expr.mptr.oper] paragraph 4:

If the dynamic type of the object does not contain the member to which the pointer refers, the behavior is undefined.

If the result of .* or ->* is a function, then that result can be used only as the operand for the function call operator (). [Example:

        (ptr_to_obj->*ptr_to_mfct)(10);

calls the member function denoted by ptr_to_mfct for the object pointed to by ptr_to_obj. ]

Andy Sawyer: Assuming the resolution is what I've assumed it is for the last umpteen years (i.e. it does the polymorphic thing), then the follow on to that is "Should there also be a way of selecting the non-polymorphic behaviour"?

Mike Miller: It might be argued that the current wording of 7.6.1.3 [expr.call] paragraph 1 does give polymorphic behavior to simple calls via pointers to members. (There is no qualified-id in obj.*pmf, and the IS says that if the function is not specified using a qualified-id, the final overrider will be called.) However, it clearly says the wrong thing when the pointer-to-member itself is specified using a qualified-id (obj.*X::pmf).

Bill Gibbons: The phrase qualified-id in 7.6.1.3 [expr.call] paragraph 1 refers to the id-expression and not to the "pointer-to-member expression" earlier in the paragraph:

For a member function call, the postfix expression shall be an implicit (11.4.3 [class.mfct.non.static] , 11.4.9 [class.static] ) or explicit class member access (7.6.1.5 [expr.ref] ) whose id-expression is a function member name, or a pointer-to-member expression (7.6.4 [expr.mptr.oper] ) selecting a function member.

Mike Miller: To be clear, here's an example:

    struct S {
	virtual void f();
    };
    void (S::*pmf)();
    void g(S* sp) {
	sp->f();         // 1: polymorphic
	sp->S::f();      // 2: non-polymorphic
	(sp->S::f)();    // 3: non-polymorphic
	(sp->*pmf)();    // 4: polymorphic
	(sp->*&S::f)();  // 5: polymorphic
    }