Re: [boost] cstring_view

On May 14, 2022, at 12:45 AM, Rainer Deyke via Boost wrote:

On 14.05.22 00:42, Marshall Clow via Boost wrote:

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On May 13, 2022, at 3:19 PM, Peter Dimov via Boost wrote:

Marshall Clow wrote:

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On May 13, 2022, at 12:29 PM, Rainer Deyke via Boost wrote:

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That's an issue with views in general, not just cstring_view.

std::string s = "hello"; string_view sv = s; assert(sv.size() == 5); s += "!"; assert(sv.size() == 5); // boom

I don’t see the problem here (and when I run the code I get no error - after adding the missing ’std::').

No assertion failure; no undefined behavior (unlike the cstring_view example)

Only because "hello!" fits into the small buffer, I suspect. If `s` reallocates, `sv` would be left dangling. Agreed. But even if the string *did* reallocate, the call "assert(sv.size() == 5)” is still valid and well defined.

No it's not. sv.size() works by subtracting pointers, and it's only legal to subtract two pointers if they point into the same memory region. Which sv.begin() and sv.end() no longer do if s reallocates. It's subtle, but it's definitely undefined behavior.

Um, no. string_view::size() *could* work by subtracting two pointers, but it’s certainly not *required* to do so. From the implementation of boost::string_view: BOOST_CONSTEXPR size_type size() const BOOST_NOEXCEPT { return len_; } From the implementation od libc++’s string_view: _LIBCPP_CONSTEXPR _LIBCPP_INLINE_VISIBILITY size_type size() const _NOEXCEPT { return __size; } From the implementation of libstdc++’s string_view: constexpr size_type size() const noexcept { return this->_M_len; } — Marshall