std::ranges::starts_with(3)

NAME
std::ranges::starts_with - std::ranges::starts_with

Synopsis
   Defined in header <algorithm>
   Call signature
   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 > (since
   requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2> (1) C++23)
   constexpr bool
       starts_with( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred =
   {},
                    Proj1 proj1 = {}, Proj2 proj2 = {} );
   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>, (since
                        ranges::iterator_t<R2>, (2) C++23)
                        Pred, Proj1, Proj2>
   constexpr bool
       starts_with( R1&& r1, R2&& r2, Pred pred = {},
                    Proj1 proj1 = {}, Proj2 proj2 = {} );
   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 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.

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 Value Std Feature
   __cpp_lib_ranges_starts_ends_with 202106L (C++23) std::ranges::starts_with,
                        std::ranges::ends_with

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <ranges>
 #include <string_view>
 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
   ranges::ends_with checks whether a range ends with another range
   (C++23) (niebloid)
   ranges::mismatch finds the first position where two ranges differ
   (C++20) (niebloid)
   starts_with checks if the string starts with the given prefix
   (C++20) (public member function of
                     std::basic_string<CharT,Traits,Allocator>)
   starts_with checks if the string view starts with the given prefix
   (C++20) (public member function of std::basic_string_view<CharT,Traits>)