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Re: [std-proposals] Fixing Inheritance of Constructors from Aggregate bases

From: Tiago Freire <tmiguelf_at_[hidden]>
Date: Sun, 7 Apr 2024 07:20:30 +0000
In many instances I wished I was able to do this:

struct Foo
   int a;
   int b = 0;

struct Bar : public Foo
  int c;

Bar var{.a = 1, .b=2, .c=3}

That’s it, none of them define explicit constructors, I just want to initialize data in my struct, I should be able to do it.

To that I have a few notes:
1. “using” keyword shouldn’t be necessary
2. you shouldn’t be allowed to do this if data is private. On your example you are changing private data, this is an anti-pattern, should not be allowed. Sure, you made the implicit constructor public, but the data members are not public.

I also thing that the following:

Bar var{1, 2, 3};

Is an abomination of the language and it should die, designated initializer is superior and I think should be used preferentially, if the proposal skips this I think it would be for the better.

My 2c

From: Std-Proposals <std-proposals-bounces_at_[hidden]> On Behalf Of David Ledger via Std-Proposals
Sent: Sunday, April 7, 2024 08:24
To: Zhihao Yuan <zy_at_[hidden]>; std-proposals_at_lists.isocpp.org
Cc: David Ledger <DavidLedger_at_[hidden]>
Subject: Re: [std-proposals] Fixing Inheritance of Constructors from Aggregate bases


Neither option A nor B would allow for designated initializers being using for non-inherited objects.

Option B proposes behaviour consistent with inheritance of constructors (for example):

    struct Foo
        Foo(int, int);

    struct Bar : Foo
        using Foo::Foo;
        int a = 0;

It would not be possible to do this:

    auto bar = Bar{1, 2, 3};

Or this:

    auto bar = Bar{Foo{1, 2}, 3};

Once the `using Foo::Foo` is applied, all data members and base classes are default initialized (except the base which the constructor was inherited from).

Kind regards,
    David Ledger
From: Zhihao Yuan <zy_at_[hidden]<mailto:zy_at_miator.net>>
Sent: Sunday, April 7, 2024 4:00 PM
To: std-proposals_at_[hidden]<mailto:std-proposals_at_[hidden]>
Cc: David Ledger <DavidLedger_at_[hidden]<mailto:DavidLedger_at_[hidden]>>
Subject: Re: [std-proposals] Fixing Inheritance of Constructors from Aggregate bases

If I have

    struct Foo { int a, b; }
    struct Bar : Foo { using Foo::Foo; int c; }

Can I do

    auto bar = Bar{.a = 1, .b = 2, .c = 3}; // ?

if so, I think it might remove some concerns
over Designated-initializers for Base Classes (open-std.org)<https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2287r2.html>

Zhihao Yuan, ID lichray
The best way to predict the future is to invent it.

On Saturday, April 6th, 2024 at 9:36 PM, David Ledger via Std-Proposals <std-proposals_at_[hidden]<mailto:std-proposals_at_[hidden]>> wrote:

Hello everybody!!
Right now, this will not compile:

struct Foo { int a, b; }

struct Bar : Foo { using Foo::Foo; }

auto bar = Bar{1, 2}; // ERROR

Which is confusing and unintuitive. This paper proposes an improvement to this behaviour.

David Ledger

Received on 2024-04-07 07:20:34