std::ranges::starts_with
From cppreference.com
| 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.
1) Let
N1 and N2 denote the size of ranges [first1, last1) and [first2, last2) respectively. If N1 < N2, returns false. Otherwise, returns true 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.2) Same as (1), but uses
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 may not be specified when calling any of them.
- None of them is visible to argument-dependent lookup.
- When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.
In practice, they may be implemented as function objects, or with special compiler extensions.
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{}; |
Notes
| Feature-test macro: | __cpp_lib_ranges_starts_ends_with |
Example
Run this code
#include <string_view> #include <algorithm> #include <iostream> #include <ranges> 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; }; constexpr auto cmp_ignore_case = [=](char8_t x, char8_t y) { return ascii_upper(x) == ascii_upper(y); }; 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) << ' ' << std::ranges::starts_with(u8"Istanbul", u8"constant"sv, {}, ascii_upper, ascii_upper) << ' ' << std::ranges::starts_with(u8"Metropolis", u8"metro"sv, cmp_ignore_case) << ' ' << std::ranges::starts_with(u8"Acropolis", u8"metro"sv, cmp_ignore_case) << '\n'; constexpr static auto v = { 1, 3, 5, 7, 9 }; constexpr auto odd = [](int x) { return x % 2; }; static_assert( std::ranges::starts_with( v, std::views::iota(1) | std::views::filter(odd) | std::views::take(3) ) ); }
Output:
true false 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>) |
| (C++20) |
checks if the string view starts with the given prefix (public member function of std::basic_string_view<CharT,Traits>) |