Title
postfixexpression->scalar_type_dtor() inconsistent
Status
cd1
Section
12.4.6 [over.ref]
Submitter
Markus Mauhart

Created on 2003-06-08.00:00:00 last changed 161 months ago

Messages

Date: 2006-04-15.00:00:00

[Voted into WP at April, 2006 meeting.]

Date: 2005-04-15.00:00:00

Proposed resolution (April, 2005):

Change 12.4.6 [over.ref] paragraph 1 as indicated:

operator-> shall be a non-static member function taking no parameters. It implements the class member access using syntax that uses ->

    postfix-expression -> templateopt id-expression
    postfix-expression -> pseudo-destructor-name

An expression x->m is interpreted as (x.operator->())->m for a class object x of type T if T::operator->() exists and if the operator is selected as the best match function by the overload resolution mechanism (12.2 [over.match]).

Date: 2005-05-01.00:00:00

Lets start with the proposed solution. In 12.4.6 [over.ref], replace line ...

postfix-expression -> id-expression
.... with the lines ...
postfix-expression -> templateopt id-expression
postfix-expression -> pseudo-destructor-name
(This then is a copy of the two lines in 7.6.1 [expr.post] covering "->dtor")

Alternatively remove the sentence "It implements class member access using ->" and the syntax line following.

Reasons:

Currently stdc++ is inconsistent when handling expressions of the form "postfixexpression->scalar_type_dtor()": If "postfixexpression" is a pointer to the scalar type, it is OK, but if "postfixexpression" refers to any smart pointer class (e.g. iterator or allocator::pointer) with class specific CLASS::operator->() returning pointer to the scalar type, then it is ill-formed; so while c++98 does allow CLASS::operator->() returning pointer to scalar type, c++98 prohibits any '->'-expression involving this overloaded operator function.

Not only is this behaviour inconsistent, but also when comparing the corresponding chapters of c++pl2 and stdc++98 it looks like an oversight and unintended result. Mapping between stdc++98 and c++pl2:

c++pl2.r.5.2 -> 5.2 [expr.post]
c++pl2.r.5.2.4 -> 5.2.4 [expr.pseudo] + 5.2.5 [expr.ref]
c++pl2.r.13.4 -> 13.3.1.2 [over.match.oper]
c++pl2.r.13.4.6 -> 13.5.6 [over.ref]
For the single line of c++pl2.r.5.2 covering "->dtor", 5.2 [expr.post] has two lines. Analogously c++pl2.r.5.2.4 has been doubled to 5.2.4 [expr.pseudo] and 5.2.5 [expr.ref]. From 13.5.6 [over.ref], the sentence forbiding CLASS::operator->() returning pointer to scalar type has been removed. Only the single line of c++pl2.r.13.4.6 (<-> c++pl2.r.5.2's single line) has not gotten its 2nd line when converted into 13.5.6 [over.ref].

Additionally GCC32 does is right (but against 13.5.6 [over.ref]).

AFAICS this would not break old code except compilers like VC7x and Comeau4301.

It does not add new functionality, cause any expression class_type->scalar_type_dtor() even today can be substituted through (*class_type).scalar_type_dtor().

Without this fix, template functions like some_allocator<T>::destroy(p) must use "(*p).~T()" or "(*p).T::~T()" when calling the destructor, otherwise the simpler versions "p->~T()" or "p->T::~T()" could be used.

Sample code, compiled with GCC32, VC7[1] and Comeau4301:

struct A {};//any class

template <class T>
struct PTR
    {
    T& operator*  () const;
    T* operator-> () const;
    };

template <class T>
void f ()
    {
        {
        T*  p               ;
        p = new T           ;
        (*p).T::~T()        ;//OK
        p = new T           ;
        (*p).~T()           ;//OK
        p = new T           ;
        p->T::~T()          ;//OK
        p = new T           ;
        p->~T()             ;//OK
        }

        {
        PTR<T> p = PTR<T>() ;
        (*p).T::~T()        ;//OK
        (*p).~T()           ;//OK
        p.operator->()      ;//OK !!!
        p->T::~T()          ;//GCC32: OK; VC7x,Com4301: OK for A; ERROR w/ int
        p->~T()             ;//GCC32: OK; VC7x,Com4301: OK for A; ERROR w/ int
        }
    }

void test ()
    {
    f <A>  ();
    f <int>();
    }
History
Date User Action Args
2008-10-05 00:00:00adminsetstatus: wp -> cd1
2006-11-05 00:00:00adminsetstatus: dr -> wp
2006-04-22 00:00:00adminsetmessages: + msg1367
2006-04-22 00:00:00adminsetstatus: ready -> dr
2005-10-22 00:00:00adminsetstatus: review -> ready
2005-05-01 00:00:00adminsetmessages: + msg1149
2005-05-01 00:00:00adminsetstatus: drafting -> review
2003-11-15 00:00:00adminsetstatus: open -> drafting
2003-06-08 00:00:00admincreate