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:
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
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] 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] 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:
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] 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.