std::is_partitioned() algorithm

since C++20

// (1)
template< class InputIt, class UnaryPredicate >
constexpr bool is_partitioned( InputIt first, InputIt last, UnaryPredicate p );

// (2)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
bool is_partitioned( ExecutionPolicy&& policy,
                     ForwardIt first, ForwardIt last, UnaryPredicate p );

since C++17

// (1)
template< class InputIt, class UnaryPredicate >
bool is_partitioned( InputIt first, InputIt last, UnaryPredicate p );

// (2)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
bool is_partitioned( ExecutionPolicy&& policy,
                     ForwardIt first, ForwardIt last, UnaryPredicate p );

since C++11

// (1)
template< class InputIt, class UnaryPredicate >
bool is_partitioned( InputIt first, InputIt 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.

  Overload Resolution

  These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (until C++20) std::is_execution_policy_v<std::is_partitioned_cvref_t<ExecutionPolicy>>  (since C++20) is true.

Parameters

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 if the element value should be replaced. 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	LegacyInputIterator
ForwardIt	LegacyForwardIterator
UnaryPredicate	Predicate

ForwardIt::value_type must be convertible to UnaryPredicate's argument type.

Return value

If the range is empty or partitioned by p - true.
Otherwise, false.

Complexity

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

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.

Possible implementation

is_partitioned (1)

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;
}

Examples

Main.cpp

#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.cbegin(), v.cend(), is_even) << ' ';
    std::cout << std::is_partitioned(v.crbegin(), v.crend(), is_even) << '\n';
}

Output

false true false true