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Re: [std-proposals] Relocation in C++

From: Sébastien Bini <sebastien.bini_at_[hidden]>
Date: Tue, 11 Oct 2022 10:35:00 +0200
Thank you for clarifying.

On Mon, Oct 10, 2022 at 3:08 PM Edward Catmur <ecatmur_at_[hidden]>

> Oh, sorry - special member function. Copy/move/relocate constructors and
> assignment operators, and destructor; the Seven. I'm not counting other
> constructors (not even the default constructor) as those don't maintain
> invariants (though they may establish them) - a class without any of the
> seven SMFs can't claim to maintain any invariants, so std::decompose should
> be safe on it. The rules would be that when calling std::decompose on a
> prvalue of class type:
> * if the class has any non-empty potentially overlapping direct subobjects
> (i.e., virtual bases or anonymous union members, but not EBO bases and
> [[no_unique_address]] members), then std::decompose is ill-formed
> * otherwise, if it has any user-declared (not declared as defaulted) SMFs,
> or if it has any private direct subobjects, then std::decompose can only be
> called by members of the class and its friends
> * otherwise, if it has any protected direct subobjects, std::decompose can
> only be called by members of the class, its friends, and its derived classes
> * otherwise, std::decompose can be called from anywhere

Hmm, should be safe enough. Requiring the same access level as member
functions if there is any user-declared SMF was the missing piece.

It is a rather atypical set of rules though. But I am okay with proposing

> And, wrt. structured binding of an object e of class type E,
> * if std::tuple_size<E>::value is well-formed, the tuple-like protocol is
> invoked to perform recursive destructuring if e is prvalue and `get_all` is
> found, else binding;
> * otherwise, if e is prvalue and neither E nor any of its base classes has
> any user-declared SMFs, data member destructuring is performed: the
> lifetime of e and its bases (but not the lifetime of its data members) is
> ended and the identifiers name the data members individually (as prvalue,
> if the data member is not ref-qualified)
> * otherwise, binding to data members occurs as at present and the
> identifiers name the data members as xvalue or lvalue

 In the second case, all data members must be from the same class and have
public access (just like data-member binding protocol in today's structured

Other than that, we should be good!

Received on 2022-10-11 08:35:13