Std::is_partitioned (3) Linux Manual Page

std::is_partitioned – std::is_partitioned

Synopsis

Defined in header <algorithm>

template< class InputIt, class UnaryPredicate >                                                             (since C++11)

bool is_partitioned( InputIt first, InputIt last, UnaryPredicate p );                                       (until C++20)

template< class InputIt, class UnaryPredicate >                                                     (1)     (since C++20)

constexpr bool is_partitioned( InputIt first, InputIt last, UnaryPredicate p );

template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >                                (2) (since C++17)

bool is_partitioned( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPredicate p );

1) Returns true if all elements in the range [first, last) that satisfy the predicate p appear before all elements that don’t. Also returns true if [first, last) is empty.
2) Same as (1), but executed according to policy. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

Parameters

first, last – the range of elements to check
policy – the execution policy to use. See execution_policy for details.
              unary predicate which returns true for the elements expected to be found in the beginning of the range.
p – The expression p(v) must be convertible to bool for every argument v of type (possibly const) VT, where VT is the value type of InputIt, regardless of value_category, and must not modify v. Thus, a parameter type of VT&is not allowed
              , nor is VT unless for VT a move is equivalent to a copy
              (since C++11).

Type requirements

–
InputIt must meet the requirements of LegacyInputIterator.
–
ForwardIt must meet the requirements of LegacyForwardIterator. and its value type must be convertible to UnaryPredicate’s argument type
–
UnaryPredicate must meet the requirements of Predicate.

Return value

true if the range [first, last) is empty or is partitioned by p. false otherwise.

Complexity

At most std::distance(first, last) applications of p.

Exceptions

The overload with a template parameter named ExecutionPolicy reports 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.

Possible implementation

  template< class InputIt, class UnaryPredicate >
  bool is_partitioned(InputIt first, InputIt last, UnaryPredicate p)
  {
      for (; first != last; ++first)
          if (!p(*first))
              break;
      for (; first != last; ++first)
          if (p(*first))
              return false;
      return true;
  }

Example

// Run this code

#include <algorithm>

#include <array>

#include <iostream>
int main()

{
    std::array<int, 9> v = {1, 2, 3, 4, 5, 6, 7, 8, 9};
    auto is_even = [](int i) { return i % 2 == 0; };
    std::cout.setf(std::ios_base::boolalpha);
    std::cout << std::is_partitioned(v.begin(), v.end(), is_even) << ' ';
    std::partition(v.begin(), v.end(), is_even);
    std::cout << std::is_partitioned(v.begin(), v.end(), is_even) << ' ';
    std::reverse(v.begin(), v.end());
    std::cout << std::is_partitioned(v.begin(), v.end(), is_even);
}

Output:

false true false

std::is_partitioned (3) Linux Manual Page