std::ranges::starts_with - cppreference.com (original) (raw)

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.

The function-like entities described on this page are algorithm function objects (informally known as 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.

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Possible implementation
• 5 Notes
• 6 Example
• 7 See also

[edit] Parameters

[edit] Return value

true if the second range matches the prefix of the first range, false otherwise.

[edit] Complexity

Linear: at most min(N1, N2) applications of the predicate and both projections.

[edit] Possible implementation

struct starts_with_fn { template<std::input_iterator I1, std::sentinel_for S1, std::input_iterator I2, std::sentinel_for 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, ranges::iterator_t, 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 {};

[edit] Notes

[edit] Example

#include #include #include #include   int main() { using namespace std::literals;   constexpr auto ascii_upper = [](char8_t c) { return u8'a' <= c && c <= u8'z' ? static_cast(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:

[edit] See also

| | checks whether a range ends with another range(algorithm function object)[edit] | | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | finds the first position where two ranges differ(algorithm function object)[edit] | | | checks if the string starts with the given prefix (public member function of std::basic_string<CharT,Traits,Allocator>) [edit] | | | checks if the string view starts with the given prefix (public member function of std::basic_string_view<CharT,Traits>) [edit] |