Defined in header <algorithm>
template< class InputIt, class UnaryPredicate >
bool is_partitioned( InputIt first, InputIt last, UnaryPredicate p );
(since C++11)
(until C++20)
template< class InputIt, class UnaryPredicate >
constexpr bool is_partitioned( InputIt first, InputIt last, UnaryPredicate p );
(since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
bool is_partitioned( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPredicate p );
(2) (since C++17)
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


first, last - the range of elements to check
policy - the execution policy to use. See execution policy for details.
p - unary predicate which returns ​true for the elements expected to be found in the beginning of the range.

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.


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


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))
    for (; first != last; ++first)
        if (p(*first))
            return false;
    return true;


#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 << 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

See also

divides a range of elements into two groups
(function template)
locates the partition point of a partitioned range
(function template)

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