std::is_partitioned
Min standard notice:
Header: <algorithm>
- Checks whether [first,last) is partitioned by the predicate p: all elements satisfy p appear before all elements that do not.
# Declarations
template< class InputIt, class UnaryPred >
bool is_partitioned( InputIt first, InputIt last, UnaryPred p );
(since C++11) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPred >
bool is_partitioned( ExecutionPolicy&& policy,
ForwardIt first, ForwardIt last, UnaryPred p );
(since C++17)
# Parameters
first, last: the range of elements to checkpolicy: the execution policy to usep: 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).
# Return value
true if the elements e of [first,last) are partitioned with respect to the expression p(e). false otherwise.
# Example
#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';
}