C++ Logo

std-proposals

Advanced search

Re: [std-proposals] Class Instance Re-Use/Re-Init

From: Scott Michaud <scott_at_[hidden]>
Date: Mon, 15 Aug 2022 14:08:16 -0400
I should note that a lot of classes can be reset just by move-assigning an
instance of the class's new state by-value.

MyClass one = MyClass(); // Intentionally constructing the verbose way.
DisturbTheStateOf(one); // Get one into whatever state.
one = MyClass(); // Pretty much resets one, albeit with some caveats (ex:
if you have a no-move, no-copy type; must be extra-careful of the order of
members when creating the class; etc.).

On Sun, Aug 14, 2022 at 8:28 PM Greg McPherran via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Hi Sebastian,
>
> >> What do you mean by "once-and-done"?
> Once a ctor is used, the ctor function is no longer available to be called
> for the existing instance. The re-init concept would be the same idea:
> Perhaps a syntax such as some_object_instance::(args) where args and
> overloads are the same as the ctor args and overloads.
>
> >> .. take the task of writing it over from you.
> Yes, this is my thought. I am requesting things in the language that I
> would find useful but I'll leave it to the language designers to see the
> value and determine the syntax. I simply wanted to place it on the table
> because I think they may be exciting. I will summarize again here:
>
>
>
> - Ctor-like re-init special functions that are not available as normal
> functions after init just as ctor is not available as a normal function on
> an object.
> - const after first init - useful for init'ing global objects that are
> intended to be constant but one would like the cnage to init them once and
> only once at runtime (startup).
> - class* pool* keyword or similar idea that causes the compiler to use
> an object pool for instances of the class.
>
>
> So these are all concepts and again I leave them for what they are worth.
> To me they have worth. If any of the language designers designers see value
> and would like to correspond, that is fine, but my goal here is to simply
> leave them on the table for the experts to decide if/how these concepts
> might be incorporated into the language.
>
> Thank You,
> Greg McPherran
>
>
> ------ Original Message ------
> From "Sebastian Wittmeier via Std-Proposals" <
> std-proposals_at_[hidden]>
> To "std-proposals_at_[hidden]" <std-proposals_at_[hidden]>
> Cc "Sebastian Wittmeier" <wittmeier_at_[hidden]>
> Date 2022-08-14 7:40:38 PM
> Subject Re: [std-proposals] Class Instance Re-Use/Re-Init
>
> Dear Greg,
>
>
>
> standardization is done by a lot of people in their free time or paid by
> their employers beside their normal work.
>
> So your task will be more of convincing people and putting effort into
> your proposals (writing the paper, researching motivations, opinions,
> solutions, previous efforts).
>
> "We all" are "C++ customers". This list will give you feedback and
> guidance. The decisions are made by ISO groups consisting of the same and
> other people according to your written proposal.
>
>
>
> If you give good reasons, it could be that other people help with your
> proposal or (in rare cases) even take the task of writing it over from you.
>
>
>
>
>
> You quickly wrote about two ideas, but did not give further details. It is
> quite difficult to understand, what you are exactly *proposing* (as you
> wrote in your first message to the list), how it would work in detail and
> what the difference (and advantages) is to solutions already possible.
>
>
>
> For the class instace-re-use/re-init proposal:
>
> - The constructors are not doing the memory operations. This comes before
> and depends:
>
> *automatic (local) variables
>
> * new
>
> * placement new
>
> * std::construct_at
>
> * std::allocator_traits::construct
>
> * std::unitialized_default/value_construct
>
> The current constructor is independent of those. It works with all ways.
> It just fills the memory, it resides in. Whether it was newly allocated or
> it is replacing existing content. What would your special function do
> different than an existing constructor, so that a new function is needed?
>
> - There are also move- and copy assignment operators as special member
> functions, which reset an existing object without destructing it first
>
> - What do you mean by "once-and-done"?
>
> - How and when would it be called? Manually? Or by the language? In what
> cases?
>
> - Everybody is free to create reset()-like functions for their classes.
> Would you make it mandatory? For all or certain classes? Would there be a
> default implementation? Which just destructs and constructs the object at
> the same place?
>
>
>
> const-after-first-init could be solved with a setter member-function,
> which only accepts data, as long as the object is not initialized yet. Or
> with std::optional again or with factory functions. Or with views or with
> Facade patterns.
>
>
> Best,
>
> Sebastian
>
>
>
>
> -----Urspr√ľngliche Nachricht-----
> *Von:* Greg McPherran via Std-Proposals <std-proposals_at_[hidden]>
> *Gesendet:* So 14.08.2022 23:17
> *Betreff:* Re: [std-proposals] Class Instance Re-Use/Re-Init
>
> So, in summary, the two features that I am requesting as a C++ customer
> are:
>
> - Ctor-like re-init once-and-done special functions that are not
> available as normal functions after init just as ctor is not available as a
> normal function on an object.
> - const after first init
>
> Thank You,
> Greg McPherran
>
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>

Received on 2022-08-15 18:08:28