# std::ranges::starts_with

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< cpp‎ | algorithm‎ | ranges

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 C++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 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 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 | Comment
--- | --- | --- | ---
`__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 (C++23) | checks whether a range ends with another range (niebloid)
ranges::mismatch (C++20) | finds the first position where two ranges differ (niebloid)
starts_with (C++20) | checks if the string starts with the given prefix (public member function of `std::basic_string<CharT,Traits,Allocator>`)
starts_with (C++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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