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Subject: Re: [std-proposals] Avoid copies when using string_view with APIs expecting null terminated strings
From: Nikolay Mihaylov (nmmm_at_[hidden])
Date: 2020-12-23 16:00:39


Sorry for comment too late, but how exactly string_view will add null at
the end, if it is const? The const char * could be in read only memory as
well.

The only way this to work is to copy the char array - into std::string or
into char array (i usually use std::array<char>.

Another way is to change your algorithm, so it does not rely on null
terminator.

Nikolay

On Wednesday, December 23, 2020, Thiago Macieira via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On Wednesday, 23 December 2020 13:48:51 -03 Arthur O'Dwyer via
> Std-Proposals
> wrote:
> > Nobody's mentioned it yet for some reason, but this is far from the first
> > time that the idea of "null-terminated string_view" has come up. The
> > traditional name for it is "zstring_view", and you can find more
> > information by googling that name.
>
> I've seen both zstring_view and cstring_view, the difference between them
> is
> that the former usually carries an explicit size whereas the latter does
> not.
> Yes, it's duplicated information, but the size is a very common
> information
> that is needed. That also makes it layout-compatible with string_view,
> whereas
> cstring_view is layout-compatible with a plain pointer.
>
> > An interesting runtime variation is bev::string_view --
> > https://github.com/lava/string_view
> > bev::string_view remembers which constructor was used to construct it --
> the
> > pointer-length constructor, or the std::string/const char* constructor? --
> > and therefore remembers whether it's safe to access the char at
> > this.data()[this.size()]. This means it knows whether it's safe to test
> > this.data()[this.size()] against '\0', which means it can frequently
> detect
> > its own null-terminated-ness at runtime. (But if it was constructed with
> > the pointer-length constructor, and never .remove_suffix'ed, then it
> would
> > conservatively report that it didn't know itself to be null-terminated
> and
> > couldn't find out without UB.)
>
> It can't report, period. Even if it could determine that the NUL is there
> after the string data, it can't guarantee that it will *remain* there.
> Since
> that byte may belong to something, it may therefore be asynchronously
> modified.
>
> The only time when it's legal to access that NUL is when doing strlen() in
> the
> constructor. And even then, this class as you described must have as part
> of
> its contract that the size()+1 must remain unchanged while the object
> lives.
>
> > Microsoft GSL used to provide (or at least, used to document) a
> > `zstring_span` with zstring_view semantics. However, these days, GSL
> > provides `czstring` only as a type alias for `const char*`, and has
> gotten
> > rid of its pre-C++17 `string_span` and `zstring_span` types.
>
> Ok, that's new to me. cstring, zstring, czstring... since it's MS, I
> suppose
> they also had lpszwstring? :-)
> --
> Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
> Software Architect - Intel DPG Cloud Engineering
>
>
>
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>



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