std::ranges::starts_with

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)
template< /execution-policy/ Ep, std::random_access_iterator I1, std::sized_sentinel_for<I1> S1, std::random_access_iterator I2, std::sized_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> bool starts_with( Ep&& policy, I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {});	(3)	(since C++26)
template< /execution-policy/ Ep, /sized-random-access-range/ R1, /sized-random-access-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> bool starts_with( Ep&& policy, R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {});	(4)	(since C++26)

For the definition of /*execution-policy*/, see this page; for the definition of /*sized-random-access-range*/, see this page.

Checks whether the target range matches the prefix of the source range. The elements (projected by proj1 and proj2 respectively) are compared using the binary predicate pred.

1) The source range is [first1, last1), and the target range is [first2, last2).

2) The source range is r1, and the target range is r2.

3,4) Same as (1,2), but executed according to policy.

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.

Parameters

first1, last1	-	the iterator-sentinel pair defining the source range
first2, last2	-	the iterator-sentinel pair defining the target range
r1	-	the source range
r2	-	the target range
pred	-	the predicate to be applied to the (projected) elements
proj1	-	the projection to be applied to the elements in the source range
proj2	-	the projection to be applied to the elements in the target range
policy	-	the execution policy to use

Return value

true if the target range matches the prefix of the source range, false otherwise.

Complexity

Given

\(\scriptsize N_1\)N₁ as ranges::distance(first1, last1) or ranges::distance(r1), and
\(\scriptsize N_2\)N₂ as ranges::distance(first2, last2) or ranges::distance(r2):

1,2) At most \(\scriptsize \min(N_1,N_2)\)min(N₁,N₂) applications of pred, proj1 and proj2.

3,4) \(\scriptsize \mathcal{O}(\min(N_1,N_2))\)𝓞(min(N₁,N₂)) applications of pred, proj1 and proj2.

Exceptions

3,4) During the execution process:

If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_ranges_starts_ends_with`	`202106L`	(C++23)	`std::ranges::starts_with`, `std::ranges::ends_with`

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 R>
    constexpr auto get_end(R&& r)
    {
        return ranges::end(r);
    }
    
    template<ranges::forward_range R>
    constexpr auto get_end(R&& r)
    {
        return ranges::next(ranges::begin(r), ranges::end(r));
    }
    
    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), get_end(r1),
                       ranges::begin(r2), get_end(r2),
                       std::move(pred), std::move(proj1), std::move(proj2));
    }
};

inline constexpr starts_with_fn starts_with{};

Example

#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

ranges::ends_with (C++23)	checks whether a range ends with another range (algorithm function object)
ranges::mismatch (C++20)	finds the first position where two ranges differ (algorithm function object)
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>`)