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Subject: Re: A contradiction between [class.base.init#9] and [class.copy.ctor#14]
From: Andrew Schepler (aschepler_at_[hidden])
Date: 2020-09-26 08:30:59


An implicitly-defined copy/move constructor can't be specified as if by a
definition with mem-initializer-list, because of array members. There's no
simple way to write the equivalent of the element-wise copy for arrays
specified in [class.copy.ctor]/(14.1)
<https://eel.is/c++draft/class.copy.ctor#14.1>.

If a clarification for this is needed, it would be simpler to change
[class.base.init]/9. For example, "In a non-defaulted, non-delegating
constructor, ..."

On Wed, Sep 23, 2020 at 4:53 AM jim x via Std-Discussion <
std-discussion_at_[hidden]> wrote:

> Hi, everyone, wish you a nice day.
>
> I have an issue about two rules in [class.base.init] and
> [class.copy.ctor], respectively. They are:
>
> [class.base.init#9] <https://eel.is/c++draft/class.base.init#9>
> In a non-delegating constructor, if a given potentially constructed
> subobject *is not designated by a mem-initializer-id* (including the case
> where there is no mem->initializer-list because the constructor has no
> ctor-initializer), then
>
>
> [class.copy.ctor#14] <https://eel.is/c++draft/class.copy.ctor#14>
> The implicitly-defined copy/move constructor for a non-union class X
> performs a memberwise copy/move of its bases and members. [Note 7: Default
> member initializers of non-static data members are ignored. See also the
> example in [class.base.init]. — end note] The order of initialization is
> the same as the order of initialization of bases and members in a *user-defined
> constructor *(see [class.base.init]). Let x be either the parameter of
> the constructor or, for the move constructor, an xvalue referring to the
> parameter. Each base or non-static data member is copied/moved in the
> manner appropriate to its type
>
>
> Except for the tips mentioned in `Note`, there's no any wording in the
> standard that says the `implicitly-defined copy/move constructor` can
> suppress the rules in [class.base.init#9] to apply to it. Due
> to `implicitly-defined copy/move constructor` has no ctor-initializer, it
> should obey [class.base.init#9]. So, it appears to be vague.
>
> So, Is it necessary to reword the rule in [class.copy.ctor#14]
> <https://eel.is/c++draft/class.copy.ctor#14> to suppress the rule
> [class.base.init#9] explicitly? Such as:
>
> The implicitly-defined copy/move constructor for a non-union class X
> performs a memberwise copy/move of its bases and members *as if it exists
> a corresponding member-initializer for each potentially constructed
> subobject*.
>
> After this modification, this rule "Virtual base class subobjects shall be
> initialized only once by the implicitly-defined copy/move constructor"
> will also be clear due to it will conform to this rule, namely "A
> mem-initializer where the mem-initializer-id denotes a virtual base class
> is ignored during execution of a constructor of any class that is not the
> most derived class." in [class.base.init#7]
> <https://eel.is/c++draft/class.base.init#7>
> Although, there's a rule in the standard to suppress that the constructor
> of non most derived class to initialize the virtual base subobject, that
> is:
> [class.base.init#13.1] <https://eel.is/c++draft/class.base.init#13.1>
> First, and only for the constructor of the most derived class
> ([intro.object]), virtual base classes are initialized in the order they
> appear on a depth-first left-to-right traversal of the directed acyclic
> graph of base classes, where “left-to-right” is the order of appearance of
> the base classes in the derived class base-specifier-list.
>
> However, it's a bit obscure, not obvious. Well, this modification is not
> completely for why the virtual base subobject only initialized once, but
> explicitly reject the rule in [class.base.init#9]
> <https://eel.is/c++draft/class.base.init#9> which may apply
> to `implicitly-defined copy/move constructor`. BTW, the sentence "The order
> of initialization is the same as the order of initialization of bases and
> members in a *user-defined constructor *(see [class.base.init])." sounds
> like that these rules in [class.base.init] is applied to `user-defined
> constructor`.
>
> Anyhow, It's only my reading. If you have any other opinions, please tell
> me, thanks.
>
>
>
>
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