std::ranges::starts_with(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

💖 NAME 💖

okay so bestie, this is the std::ranges::starts_with(3) page from the C++ Standard Libary (yes, the header literally spells it “Libary” — iconic, no notes). The thing itself is std::ranges::starts_with — yes, name so nice it said it twice: std::ranges::starts_with - std::ranges::starts_with. It’s the “does range two match the beginning of range one?” girly. periodt. 💅

✨ Synopsis ✨

Defined in header <algorithm> — call signature incoming ✨

Defined in header <algorithm>
Call signature

Overload (1) — since C++23 🎀

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 = {} );

Overload (2) — 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 = {} );

So what does it do? Checks whether the second range matches the prefix of the first range. That’s the whole vibe check. 🫶

  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. lowkey important, bestie 👀

🌸 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. that’s it. that’s the answer. ✨

🦋 Complexity 🦋

Linear: at most min(N1, N2) applications of the predicate and both projections. no secret extra chaos, we love a clean bound. 💜

💅 Possible implementation 💅

here’s what a possible implementation looks like — it’s giving function-object realness:

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 check 🕵️‍♀️ — yes, there’s a macro for this:

  • Feature-test macro: __cpp_lib_ranges_starts_ends_with
  • Value: 202106L
  • Std: (C++23)
  • Feature: 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>)

mwah 💋 this page says it came from http://cppreference.com, dated 2024.06.10, and the page is std::ranges::starts_with(3). bye bestie 🫶

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