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Re: [std-proposals] Contradiction about this parameter in the C++ standard

From: language.lawyer_at <language.lawyer_at_[hidden]>
Date: Tue, 19 Apr 2022 11:46:56 +0300
On 19/04/2022 11:24, Edward Catmur wrote:
> On Tue, 19 Apr 2022 at 08:57, <language.lawyer_at_[hidden]> wrote:
>> On 19/04/2022 10:37, Edward Catmur wrote:
>>> On Tue, 19 Apr 2022 at 03:22, <language.lawyer_at_[hidden]> wrote:
>>>> On 18/04/2022 23:23, Edward Catmur wrote:
>>>>> On Mon, 18 Apr 2022 at 21:13, <language.lawyer_at_[hidden]> wrote:
>>>>>> On 18/04/2022 22:57, Edward Catmur wrote:
>>>>>>> I suppose that paragraph <
>>>> https://eel.is/c++draft/expr.call#7.sentence-4>
>>>>>> (and
>>>>>>> the following Note) should be struck entirely.
>>>>>> I'd say then we will not know which object [expr.prim.this]/1 speaks
>>>> about
>>>>>> («The keyword `this` names a pointer to the object for which an
>> implicit
>>>>>> object member function is invoked»)
>>>>> "the object for which an implicit object member function is invoked"
>>>> seems
>>>>> clear enough; per [expr.call]/2 it's the object expression on the LHS
>> of
>>>>> the postfix expression (before the '.', '->', '.*' or '->*').
>>>> If it is a base-class function, the object is the corresponding base
>> class
>>>> subobject, isn't it?
>>> Yes, but that's a problem with [expr.prim.this]/1; you can't have a
>> pointer
>>> of base type pointing to an object of derived type. So that subclause is
>>> also defective, and should have appended ", or to its base class
>> subobject,
>>> as appropriate".
>> I don't understand where the defect is.
>> When «the object for which function is invoked» is a base class subobject
>> (and it is pointed to by `this`), there is no discrepancy between the type
>> of `this` and the object's type.
>> «The object for which function is invoked» does not have to be «the result
>> of the object expression».
> If it isn't (and is the base class subobject), where is that specified? Or
> where should it be specified?

[expr.call]/7 tries to specify this. https://github.com/cplusplus/draft/pull/5399

>> As for [expr.call]/7, there's no need to specify *how* the implementation
>>> is expected to come up with the value of the `this` pointer (although it
>>> could be a Note).
>> When you say that [expc.call]/2 defines the object for which the function
>> is invoked, do you mean «the call is as a member of the class object
>> referred to by the object expression»?
> Yes, I think that's where the connection is made. So that's possibly a
> derived class object, and it's worth explaining where the this pointer with
> type pointer to base gets its value from.

IDK, why/when having «the object for which function is invoked» meaning «the result of the object expression» and overloading the definition of `this` with «or to its base class subobject» is useful?

Received on 2022-04-19 08:47:00