std::find_end

class ForwardIt1, class ForwardIt2, class BinaryPred >
ForwardIt1 find_end( ExecutionPolicy&& policy,
ForwardIt1 first, ForwardIt1 last,
ForwardIt2 s_first, ForwardIt2 s_last,

BinaryPred p );

(4) (since C++17)

Searches for the last occurrence of the sequence [s_first, s_last) in the range [first, last).

1) Elements are compared using operator==.

3) Elements are compared using the given binary predicate p.

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

These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v <std::decay_t <ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v <std::remove_cvref_t <ExecutionPolicy>> is true.

(since C++20)

While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)).
The types Type1 and Type2 must be such that objects of types ForwardIt1 and ForwardIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively.

Type requirements

-ForwardIt1 must meet the requirements of LegacyForwardIterator.

-ForwardIt2 must meet the requirements of LegacyForwardIterator.

[edit] Return value

Iterator to the beginning of last occurrence of the sequence [s_first, s_last) in range [first, last).

If [s_first, s_last) is empty or if no such sequence is found, last is returned.

[edit] Complexity

Given \(\scriptsize N\)N as std::distance (first, last) and \(\scriptsize S\)S as std::distance (s_first, s_last):

1,2) At most \(\scriptsize S\cdot(N-S+1)\)S·(N-S+1) comparisons using operator==.

3,4) At most \(\scriptsize S\cdot(N-S+1)\)S·(N-S+1) applications of the predicate p.

[edit] Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Possible implementation

find_end (1)

template<class ForwardIt1, class ForwardIt2>
constexpr //< since C++20
ForwardIt1 find_end(ForwardIt1 first, ForwardIt1 last,
 ForwardIt2 s_first, ForwardIt2 s_last)
{
 if (s_first == s_last)
 return last;
 
 ForwardIt1 result = last;
 while (true)
 {
 ForwardIt1 new_result = std::search (first, last, s_first, s_last);
 if (new_result == last)
 break;
 else
 {
 result = new_result;
 first = result;
 ++first;
 }
 }
 return result;
}

find_end (3)

template<class ForwardIt1, class ForwardIt2, class BinaryPred>
constexpr //< since C++20
ForwardIt1 find_end(ForwardIt1 first, ForwardIt1 last,
 ForwardIt2 s_first, ForwardIt2 s_last,
 BinaryPred p)
{
 if (s_first == s_last)
 return last;
 
 ForwardIt1 result = last;
 while (true)
 {
 ForwardIt1 new_result = std::search (first, last, s_first, s_last, p);
 if (new_result == last)
 break;
 else
 {
 result = new_result;
 first = result;
 ++first;
 }
 }
 return result;
}

[edit] Example

Run this code

#include <algorithm>
#include <array>
#include <cmath>
#include <iostream>
 
auto print_result = [](auto result, const auto& v)
{
 result == v.end()
 ? std::cout << "Sequence not found\n"
 : std::cout << "Last occurrence is at: " << std::distance (v.begin(), result)
 << '\n';
};
 
int main()
{
 const auto v = {1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4};
 
 for (auto const& x : {std::array {1, 2, 3}, {4, 5, 6}})
 {
 auto iter = std::find_end(v.begin(), v.end(), x.begin(), x.end()); // overload (1)
 print_result(iter, v);
 }
 
 for (auto const& x : {std::array {-1, -2, -3}, {-4, -5, -6}})
 {
 auto iter = std::find_end(v.begin(), v.end(), x.begin(), x.end(), // overload (3)
 [](int x, int y)
 {
 return std::abs(x) == std::abs(y);
 });
 print_result(iter, v);
 }
}

Output:

Last occurrence is at: 8
Sequence not found
Last occurrence is at: 8
Sequence not found

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 1205	C++98	the return value was unclear if `[`s_first`,` s_last`)` is empty	returns last in this case
LWG 2150	C++98	the condition of "sequence occurence" was incorrect	corrected