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Subject: Re: [std-proposals] Attribute view
From: Andrew Tomazos (andrewtomazos_at_[hidden])
Date: 2019-12-19 13:09:33


I think this is a problem worth solving. Its against a C++ design
principle that built-in references (like const std;:string&) have the
lifetime extension feature, whereas user-defined reference-like classes
(like std::string_view) do not. In particular, this is one the arguments
against migrating from const std::string& to std::string_view.

I encourage you to submit a proposal for C++23:
https://isocpp.org/std/submit-a-proposal

On Thu, Dec 19, 2019 at 8:44 AM Martin Küttler via Std-Proposals <
std-proposals_at_[hidden]> wrote:

>
> Hi,
>
> first, merry Christmas to all who care. To the rest, merry next week.
>
> Secondly, I'd like to propose an attribute view (or called
> differently). This is my first attempt to advertise this, so feel free
> to redirect me. I understand that visiting a meeting is recommended
> before such a proposal. Unfortunately, my health does not allow me to do
> so.
>
> One of the main issues of std::string_view is ilustrated by the
> following example.
>
> std::string const hello = "hello";
> std::string_view s = hello + ", world";
> // access of s is UB
>
> Interestingly, this problem does not occur in the following code.
>
> std::string const hello = "hello";
> std::string const & s = hello + ", world";
> // access of s is fine
>
> Intuitively, this should result in the same problem: a reference to a
> temporary is taken. But in [class.temporary] it is specified that
> ``when a reference is bound to a temporary object [...] the
> temporary object that is the complete object of a subobject to which
> the reference is bound persists for the lifetime of the reference
> if [...].'' This does not apply if the temporary is bound to a T
> const & argument of a constructor. The presence of such an argument
> does not imply the need for this mechanism, as the argument might be
> used to copy a subobject, call const member functions, or might be
> discarded.
>
> The proposed solution is to add a new attribute, called view in the
> following, to the argument, that indicates that the extension of the
> lifetime is desired. The attribute is needed in the declaration (as it
> changes behaviour at the call site). Requiring consistancy between all
> declarations and the declarations and the definition is optional, but
> assumed here. Adding this attribute has two positive effects at the
> call site:
>
> - The lifetime of temporaries can be increased appropreately. This
> reduces the occurance of dangling references.
>
> - A diagnostic message can be issued if the compiler can not
> defer the destruction of the temporary long enough, e.g. when an
> object constructed with a temporary bound to a view argument is
> returned.
>
> The second point makes this an interesting feature to allow for T&, T*
> and T const * as well.
>
> In the definition diagnostics indicating the need of an added view
> are possible. This is the case e.g. if a reference to a subobject of
> an argument is stored in a constructor, or if an owner pointer is
> copied. This is not completely investigated yet, but additional
> diagnostics can be added later.
>
> The motivation for allowing view arguments in other functions but
> constructors is twofold.
>
> - Starting in C++20, a function might be a coroutine, outliving
> the point it is called from. According lifetime is required from its
> arguments.
>
> - An argument to a normal function might be used to construct an
> object that outlives the function, e.g. by being returned. In this
> case, the arguments should be marked view recursively. Technically
> speaking this case probably includes the coroutine-case. Diagnostic
> messages for arguments used to construct view parameters to other
> calls, that are not themselves marked view, should be required.
>
> Implications for old code
> -------------------------
>
> Code that does not use the attribute view is not impeded. Additional
> diagnostics might ensue, but no behavioral change is required from
> this code.
>
> Usages in the standard library
> ------------------------------
>
> The attribute should be added to all views (std::string_view, std::span
> and some in std::ranges::views, I think) to the constructors taking a
> std::string const & or a char const* in the first case, and a
> std::array<T, n> const & or a T (&)[n] in the second case. std::span has
> a hand full of constructors, some of which need the attribute as
> well. In addition the copy constructors of both classes need the
> attribute. The same probably holds for many constructors of most of the
> classes in std::ranges::views, but I'm no expert there.
>
> Conditional view
> ----------------
>
> To accommodate "view-correct" template code, a form of conditional
> view-ness is needed. This requires a syntax to request the presence of
> the argument at the n-th parameter of a specific function. I do not yet
> have decided how that should look.
>
> Regards,
> Martin Küttler
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>



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