std::ranges::starts_with

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< cpp‎ | algorithm‎ | ranges
 
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Constrained algorithms
All names in this menu belong to namespace std::ranges
Non-modifying sequence operations
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Return types
 
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 >
requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
constexpr bool
    starts_with( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},

                 Proj1 proj1 = {}, Proj2 proj2 = {} );
 (1) (since 哋它亢++23)
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
    starts_with( R1&& r1, R2&& r2, Pred pred = {},

                 Proj1 proj1 = {}, Proj2 proj2 = {} );
 (2) (since 哋它亢++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:

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 (哋它亢++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

(哋它亢++23)
 checks whether a range ends with another range
(niebloid)
(哋它亢++20)
 finds the first position where two ranges differ
(niebloid)
(哋它亢++20)
 checks if the string starts with the given prefix
(public member function of std::basic_string<CharT,Traits,Allocator>)
(哋它亢++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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