Difference between revisions of "cpp/iterator/unreachable sentinel t"
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Andreas Krug (Talk | contribs) m (fmt, headers sorted) |
m (fmt.) |
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{{dsc end}} | {{dsc end}} | ||
| − | {{member|operator{{==}} | + | {{member|operator{{==}}{{small|(std::unreachable_sentinel_t)}}| |
{{ddcl|since=c++20|1= | {{ddcl|since=c++20|1= | ||
template<std::weakly_incrementable I> | template<std::weakly_incrementable I> | ||
| Line 41: | Line 41: | ||
template<class CharT> | template<class CharT> | ||
| − | constexpr std::size_t strlen(const CharT *s) | + | constexpr std::size_t strlen(const CharT* s) |
{ | { | ||
return std::ranges::find(s, std::unreachable_sentinel, CharT{}) - s; | return std::ranges::find(s, std::unreachable_sentinel, CharT{}) - s; | ||
| Line 47: | Line 47: | ||
template<class CharT> | template<class CharT> | ||
| − | constexpr std::size_t | + | constexpr std::size_t find_first(const CharT* haystack, const CharT* needle) |
{ | { | ||
| + | const char* needle_end = needle + strlen(needle); | ||
// search(begin, unreachable_sentinel) is usually more efficient than | // search(begin, unreachable_sentinel) is usually more efficient than | ||
// search(begin, end) due to one less comparison per cycle. | // search(begin, end) due to one less comparison per cycle. | ||
| − | // But "needle" MUST BE in the "haystack", otherwise the call | + | // But "needle" MUST BE PRESENT in the "haystack", otherwise the call |
| − | // which is a compile-time error in constexpr context. | + | // is UB (which is a compile-time error in constexpr context). |
| − | + | auto found = std::ranges::search(haystack, std::unreachable_sentinel, | |
| − | + | needle, needle_end); | |
| − | + | return found.begin() - haystack; | |
| − | + | ||
} | } | ||
int main() | int main() | ||
{ | { | ||
| − | static_assert(strlen("The quick brown fox jumps over | + | static_assert(strlen("The quick brown fox jumps over a lazy dog.") == 42); |
| − | static_assert( | + | static_assert(find_first("const short int", "short") == 6); |
| − | // static_assert( | + | // static_assert(find_first("long int", "float")); // compile-time error |
} | } | ||
}} | }} | ||
Revision as of 03:46, 17 July 2024
| Defined in header <iterator>
|
||
| struct unreachable_sentinel_t; |
(1) | (since C++20) |
| inline constexpr unreachable_sentinel_t unreachable_sentinel{}; |
(2) | (since C++20) |
1)
unreachable_sentinel_t is an empty class type that can be used to denote the “upper bound” of an unbounded interval.2)
unreachable_sentinel is a constant of type unreachable_sentinel_t.Contents |
Non-member functions
| operator== (C++20) |
compares an unreachable_sentinel_t with a value of any weakly_incrementable type (function template) |
operator==(std::unreachable_sentinel_t)
| template<std::weakly_incrementable I> friend constexpr bool operator==( unreachable_sentinel_t, const I& ) noexcept |
(since C++20) | |
unreachable_sentinel_t can be compared with any weakly_incrementable type and the result is always false.
This function template is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::unreachable_sentinel_t is an associated class of the arguments.
Example
Run this code
#include <algorithm> #include <cstddef> #include <iostream> #include <iterator> template<class CharT> constexpr std::size_t strlen(const CharT* s) { return std::ranges::find(s, std::unreachable_sentinel, CharT{}) - s; } template<class CharT> constexpr std::size_t find_first(const CharT* haystack, const CharT* needle) { const char* needle_end = needle + strlen(needle); // search(begin, unreachable_sentinel) is usually more efficient than // search(begin, end) due to one less comparison per cycle. // But "needle" MUST BE PRESENT in the "haystack", otherwise the call // is UB (which is a compile-time error in constexpr context). auto found = std::ranges::search(haystack, std::unreachable_sentinel, needle, needle_end); return found.begin() - haystack; } int main() { static_assert(strlen("The quick brown fox jumps over a lazy dog.") == 42); static_assert(find_first("const short int", "short") == 6); // static_assert(find_first("long int", "float")); // compile-time error }
See also
| (C++20) |
a view consisting of a sequence generated by repeatedly incrementing an initial value(class template) (customization point object) |
