std::is_partitioned

`<algorithm>`
	(1)
`template< class InputIt, class UnaryPredicate > bool is_partitioned( InputIt first, InputIt last, UnaryPredicate p );`			(C++11) (C++20)
`template< class InputIt, class UnaryPredicate > constexpr bool is_partitioned( InputIt first, InputIt last, UnaryPredicate p );`			(C++20)
`template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate > bool is_partitioned( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPredicate p );`		(2)	(C++17)

1) [first, last) p true [first, last) true

2) (1) policy std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> true

first, last	-
policy	-
p	-	`true` `p(v)` `VT` ( const ) `v` `bool` `VT` `InputIt` `v` `VT&` VT `VT` (C++11)

- `InputIt` LegacyInputIterator
- `ForwardIt` LegacyForwardIterator UnaryPredicate's
- `UnaryPredicate` Predicate

[first, last) p true false

std::distance(first, last) p

ExecutionPolicy

ExecutionPolicy std::terminate ExecutionPolicy
std::bad_alloc

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;
}

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);
}

:

false true false

partition	2 () [edit]
partition_point (C++11)	() [edit]

cppreference.com

std::is_partitioned