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std::ranges::starts_with - cppreference.com

Call signature

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.

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

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

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 {};

#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:

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