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Subject: Re: [std-proposals] Avoid copies when using string_view with APIs expecting null terminated strings
From: Tom Mason (wheybags_at_[hidden])
Date: 2020-12-23 04:16:46


I do think it's useful to use the one type. For example you might want to pass a file path to some image loading code. You support several different image formats, and some of the loaders are plain c libraries that expect a null terminated path. You check the file extension by iterating from the end of the string until you find a ".", then pass the string into the appropriate library. If you didn't have the length available too, then you would need an strlen before you start that backwards iteration.
I know that you could use std::filesystem::path for that, but I mean it as a representative sample (also you might not want to force callers to copy into a path).

Another example is if you're doing some normal modern c++ code and you just want to printf something along the way because printf is still pretty convenient.

23 Dec 2020 5:22:30 am Jason McKesson via Std-Proposals <std-proposals_at_[hidden]>:

> On Tue, Dec 22, 2020 at 8:13 PM Thiago Macieira via Std-Proposals
> <std-proposals_at_[hidden]> wrote:
>>
>> On Tuesday, 22 December 2020 21:33:35 -03 Tom Mason via Std-Proposals wrote:
>>> The problem is that there is no way to know if a given string_view is null
>>> terminated. My proposal is to add a method bool null_terminated() to
>>> string_view. When a string_view is constructed, the caller can tell the
>>> implementation that the pointer it is providing is null terminated. The
>>> conversion from std::string to std::string_view would set this flag.
>>
>> This is an ABI-changing proposal. Please provide strong enough motivation to
>> warrant it.
>>
>> Given the cost, it might be worth to have a separate class to indicate zero-
>> terminated string views.
>
> It should also be noted that if you have an API that needs a
> NUL-terminated string view... then it *needs* a NUL-terminated string
> view. That strongly suggests a different type, as that's how we
> typically spell such needs in APIs. That way, users using your API can
> see what it needs and can provide what you need. They can even
> propagate that need up the call graph.
>
> In any case, the thing about such a view is that it should just be a
> `char const *` wrapped in an object. It shouldn't have a size member,
> and it shouldn't be convertible to/from `string_view`. Also, its range
> begin/end functions should be an iterator/sentinel pair, not a pair of
> iterators. The sentinel would do the NUL check. I wrote a skeletal
> version of such a class and found that compilers were pretty good
> about optimizing uses of it down to the equivalent of common C-string
> loops.
> --
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