Difference between revisions of "cpp/algorithm/ranges/starts with"
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template<ranges::input_range R1, ranges::input_range R2, | template<ranges::input_range R1, ranges::input_range R2, | ||
class Pred = ranges::equal_to, | class Pred = ranges::equal_to, | ||
Revision as of 02:12, 14 June 2021
| Defined in header <algorithm>
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| Call signature |
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| template<std::input_iterator I1, std::sentinel_for<I1> S1, std::input_iterator I2, std::sentinel_for<I2> S2, |
(1) | (since C++23) |
| template<ranges::input_range R1, ranges::input_range R2, class Pred = ranges::equal_to, |
(2) | (since C++23) |
Checks whether the second range matches the prefix of the first range.
N1 and N2 denote the size of ranges [first1, last1) and [first2, last2) respectively. If N1 < N2, returns false. Otherwise, returns true if only if every element in the range [first2, last2) is equal to the corresponding element in [first1, first1 + N2). Comparison is done by applying the binary predicate pred to elements in two ranges projected by proj1 and proj2 respectively.r1 and r2 as the source ranges, as if using ranges::begin(r1) as first1, ranges:begin(r2) as first2, ranges::end(r1) as last1, and ranges::end(r2) as last2.The function-like entities described on this page are niebloids, that is:
- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.
In practice, they may be implemented as function objects, or with special compiler extensions.
Contents |
Parameters
| first1, last1 | - | the range of elements to examine |
| r1 | - | the range of elements to examine |
| first2, last2 | - | the range of elements to be used as the prefix |
| r2 | - | the range of elements to be used as the prefix |
| pred | - | the binary predicate that compares the projected elements |
| proj1 | - | the projection to apply to the elements of the range to examine |
| proj2 | - | the projection to apply to the elements of the range to be used as the prefix |
Return value
true if the second range matches the prefix of the first range, false otherwise.
Complexity
Linear: at most min(N1, N2) applications of the predicate and both projections.
Possible implementation
struct starts_with_fn { template<std::input_iterator I1, std::sentinel_for<I1> S1, std::input_iterator I2, std::sentinel_for<I2> S2, class Pred = ranges::equal_to, class Proj1 = std::identity, class Proj2 = std::identity> requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2> constexpr bool operator()(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const { return ranges::mismatch(std::move(first1), last1, std::move(first2), last2, std::move(pred), std::move(proj1), std::move(proj2) ).in2 == last2; } template<ranges::input_range R1, ranges::input_range R2, class Pred = ranges::equal_to, class Proj1 = std::identity, class Proj2 = std::identity> requires std::indirectly_comparable<ranges::iterator_t<R1>, ranges::iterator_t<R2>, Pred, Proj1, Proj2> constexpr bool operator()(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const { return (*this)(ranges::begin(r1), ranges::end(r1), ranges::begin(r2), ranges::end(r2), std::move(pred), std::move(proj1), std::move(proj2)); } }; inline constexpr starts_with_fn starts_with{}; |
Example
#include <string_view> #include <algorithm> #include <iostream> int main() { using namespace std::literals; constexpr auto ascii_upper = [](char8_t c) { return u8'a' <= c && c <= u8'z' ? static_cast<char8_t>(c + u8'A' - u8'a') : c; }; static_assert(std::ranges::starts_with("const_cast", "const"sv)); static_assert(std::ranges::starts_with("constexpr", "const"sv)); static_assert(!std::ranges::starts_with("volatile", "const"sv)); std::cout << std::boolalpha << std::ranges::starts_with(u8"Constantinopolis", u8"constant"sv, {}, ascii_upper, ascii_upper) << '\n' << std::ranges::starts_with(u8"Istanbul", u8"constant"sv, {}, ascii_upper, ascii_upper) << '\n'; }
Output:
true false
See also
| (C++23) |
checks whether a range ends with another range (niebloid) |
| (C++20) |
finds the first position where two ranges differ (niebloid) |
| (C++20) |
checks if the string starts with the given prefix (public member function of std::basic_string<CharT,Traits,Allocator>)
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| (C++20) |
checks if the string view starts with the given prefix (public member function of std::basic_string_view<CharT,Traits>)
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