std::ranges::starts_with() algorithm

since C++20

Simplified

// (1)
constexpr bool starts_with( I1 first1, S1 last1, I2 first2, S2 last2,
                            Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );

// (2)
constexpr bool starts_with( R1&& r1, R2&& r2,
                            Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );

The type of arguments are generic and have the following constraints:

• I1, I2 - std::input_iterator
• S1, S2 - std::sentinel_for<I1>, std::sentinel_for<I2>
• Pred - (none)
• Proj1, Proj2 - (none)
• (2) - R1, R2 - std::ranges::input_range

The Pred template argumenth as a default type of ranges::equal_to for all overloads.
The Proj1 and Proj2 template arguments have a default type of std::identity for all overloads.

Additionally, each overload has the following constraints:

• (1) - indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
• (2) - indirectly_comparable<ranges::iterator_t<R1>, ranges::iterator_t<R2>, Pred, Proj1, Proj2>

(The std:: namespace was ommitted here for readability)

Detailed

// (1)
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 = {} );

// (2)
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 = {} );

Checks whether the second range matches the prefix of the first range.

• (1) 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 as the first source range and r2 as the second source range, as if using ranges::begin(r1) as first1, ranges::end(r1) as last1, ranges::begin(r2) as first2, and ranges::end(r2) as last2.

The function-like entities described on this page are niebloids.

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 prefix.
r2	The range of elements to be used as prefix.
pred	Binary predicate to compare the elements with.
proj1	Projection to apply to the elements in the first range.
proj2	Projection to apply to the elements in the second range.

Return value

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

More specifically:

Let N1 and N2 denote the size of ranges [first1; last1) and [first2; last2) respectively.

• If N1 < N2, returns false.
• Otherwise, if every element in the range [first2; last2) is equal to the corresponding element in [first1; first1 + N2) (compared using pred after projections), returns true.

Complexity

At most min(N1, N2) applications of the predicate and both projections.

Exceptions

(none)

Possible implementation

starts_with(1) and starts_with(2)

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 {};

Examples

Main.cpp

#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