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Re: [std-proposals] Relax condition for potentially invoked destructor in constructor

From: Arthur O'Dwyer <arthur.j.odwyer_at_[hidden]>
Date: Fri, 25 Feb 2022 10:57:08 -0500
On Fri, Feb 25, 2022 at 1:57 AM Chris Ryan via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> >" The choice of allowing you to specify members in the member
> initialization list out of order from their order of initialization?
> Honestly, I have no idea why that was ever legal."
> Quickly looking I could not find the reference for it but I did not think
> that it was ever legal.
> Most every compiler I tested (on godbolt) did report it as a warning
> (-Wreorder) with -Wall or /Wall.
> "Initializer order does not match the declaration order" or something
> like that depending on your compiler

Yes, but the reason that the compiler *gives* you a warning is because the
construct is (for some reason) legal C++ that the compiler must accept
(despite being never-what-you-wanted). If the construct were actually
forbidden by the standard, then you'd get an error instead, even without

(Also, pedantic nit: IIRC, I believe MSVC's equivalent of -Wall is -W4;
their -Wall is actually the equivalent of Clang's -Weverything.)

You [can] have many constructors [...] but only one destructor and it needs
> to use the reverse order (of declaration).
> If the destructor must be in reverse order of declaration, construction
> must be in order of declaration.

Yup, exactly!

I vaguely wonder if the original reason for permitting the programmer to
    struct S {
        explicit S() : b(1), a(2) {}
        int a;
        int b;
(and the compiler to silently reorder the initializers) was somehow also
related to C++'s permission for the compiler to silently reorder the
*layout* of `a` and `b` in memory (because this is not a standard-layout
class). No compiler has ever actually done that, and IIRC the permission to
reorder memory layouts has been proposed-to-be-revoked and maybe even
scaled back in C++23.
It's kind of the same idea, from the 1990s point of view: Instead of
forcing the programmer to write everything in the "correct" order like bad
old C, let's *permit* the programmer to write in whatever order they like,
and then the compiler can magically fix it up! Hooray C++!
It's only after like 20 years of that stuff that enough people are publicly
saying "well, this is confusing; we should just force the programmer to
write what they mean."


Received on 2022-02-25 15:57:20