The requirements are perfectly well-defined. I'll be working with the N4868 draft of the C++20 standard [0]. First, `std::string::string(s)` is equivalent to `s

The requirements are perfectly well-defined. I'll be working with the N4868 draft of the C++20 standard [0]. First, `std::string::string(s)` is equivalent to `std::string::string(s, std::char_traits<char>::length(s))` (§21.3.3.3). Next, `std::char_traits<char>::length(s)` is equal to the smallest `size_t` value `i` such that `s[i] == 0` (§21.2.2, §21.2.4.2). From here, we yield our only preconditions for the call being valid: `s` must point to either a `char` equal to 0 or an element of an array of `char`s that is eventually followed by a 0 (§7.6.1.2, §7.6.6), and there must be no thread or signal handler concurrently modifying it (which would cause a data race, §6.9.2.2). These are necessary for each `s[i]` access in the length computation to be valid. Letting `n` be the computed length, we have our final precondition, that `[s, s + n)` is a valid range (§21.3.3.3). "Valid range" has a specific meaning in the standard (§23.3.1): > A sentinel `s` is called reachable from an iterator `i` if and only if there is a finite sequence of applications of the expression `++i` that makes `i == s`. If `s` is reachable from `i`, `[i, s)` denotes a valid range. Since `s` must point to an array of `char`s of length greater than `n`, `s + n` is clearly reachable from `s`. Therefore, `[s, s + n)` denotes a valid range, given our preconditions. The only length restriction in the standard is that the source string must be no more than `SIZE_MAX` characters long (excluding the null terminator), since otherwise `std::char_traits<char>::length(s)` would have no correct return value. [0] https://open-std.org/jtc1/sc22/wg21/docs/papers/2020/n4868.pdf https://open-std.org/jtc1/sc22/wg21/docs/papers/2020/n4868.p...