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Re: [SG16-Unicode] [isocpp-lib-ext] Proposed design change to P1030 filesystem::path_view

From: Victor Zverovich <victor.zverovich_at_[hidden]>
Date: Mon, 26 Aug 2019 13:46:44 -0700
> Unfortunately, that won't fix any of the bugs in your original code

It will fix the most important issue (stack overflow). The nonsensical
presentation is irrelevant to the problem illustrated by that example. That
said, I think that either way of fixing the issue is fine.

- Victor

On Mon, Aug 26, 2019 at 12:29 PM Arthur O'Dwyer via Lib-Ext <
lib-ext_at_[hidden]> wrote:

> On Mon, Aug 26, 2019 at 6:21 AM Niall Douglas via Lib-Ext <
> lib-ext_at_[hidden]> wrote:
>> On 26/08/2019 02:14, Arthur O'Dwyer wrote:
>> > A single path is not conceptually a "range" of /*anything*/ — it's just
>> > a single filesystem path.
>> filesystem::path defines that it is a range of filesystem path
>> components. Apart from the native path separator being a valid character
>> in a path component on some systems, I find that a reasonable design
>> choice.
> Well, that;
> and that you lose the separators *between* components;
> and that you can't tell the difference between a drive letter and a path
> component;
> and that your sample code tries to "std::cout <<" paths, which doesn't
> work because every `path` object is implicitly wrapped in `std::quoted`.
> https://godbolt.org/z/lpQA61
> A path *itself* is not a range of anything — it's just a path.
> std::filesystem::path breaks this intuition by giving
> std::filesystem::path ".begin()" and ".end()" methods, which causes C++ to
> treat it like a range type. But that's just an example of "lying to the
> compiler." Giving .begin() and .end() methods to a C++ class type that does
> not itself *represent* a range of elements was the original sin.
> > What it should have done was to give `std::filesystem::path` one or more
>> > "range accessor" methods:
>> > for (auto&& component : mypath.components()) { ... }
>> > for (auto&& character : mypath.str()) { ... }
>> Or iteration returns a path_component type, as Victor suggested.
> Unfortunately, that won't fix any of the bugs in your original code
> sample. You'll still lose the separators between components, and the drive
> letter, and emit many spurious quotation marks. To fix your original code
> sample, you need some way to tell your generic code that a
> std::filesystem::path should never be treated as a range.
> However, your original code sample is broken anyway for things like
> `std::vector<int>` — it prints {1,2} and {12} identically.
> Disabling path component iteration I don't think is worth the pain,
>> personally.
> I don't see any pain except the pain of people trying to use `for (auto&&
> elt : path)` in production. Stopping that erroneous construct from
> compiling would be a *good thing*.
> –Arthur
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Received on 2019-08-26 22:46:57