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Showing content from https://cplusplus.com/reference/algorithm/is_partitioned/ below:

function template

<algorithm>

std::is_partitioned 
template <class InputIterator, class UnaryPredicate>  bool is_partitioned (InputIterator first, InputIterator last, UnaryPredicate pred);

Test whether range is partitioned

Returns true if all the elements in the range [first,last) for which pred returns true precede those for which it returns false.

If the range is empty, the function returns true.

The behavior of this function template is equivalent to:


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template <class InputIterator, class UnaryPredicate>
  bool is_partitioned (InputIterator first, InputIterator last, UnaryPredicate pred)
{
  while (first!=last && pred(*first)) {
    ++first;
  }
  while (first!=last) {
    if (pred(*first)) return false;
    ++first;
  }
  return true;
}

Parameters
first, last
Input iterators to the initial and final positions of the sequence. The range used is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
pred
Unary function that accepts an element in the range as argument, and returns a value convertible to bool. The value returned indicates whether the element belongs to the first group (if true, the element is expected before all the elements for which it returns false).
The function shall not modify its argument.
This can either be a function pointer or a function object.

Return valuetrue if all the elements in the range [first,last) for which pred returns true precede those for which it returns false.
Otherwise it returns false.

If the range is empty, the function returns true.



Example 
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// is_partitioned example
#include <iostream>     // std::cout
#include <algorithm>    // std::is_partitioned
#include <array>        // std::array

bool IsOdd (int i) { return (i%2)==1; }

int main () {
  std::array<int,7> foo {1,2,3,4,5,6,7};

  // print contents:
  std::cout << "foo:"; for (int& x:foo) std::cout << ' ' << x;
  if ( std::is_partitioned(foo.begin(),foo.end(),IsOdd) )
    std::cout << " (partitioned)\n";
  else
    std::cout << " (not partitioned)\n";

  // partition array:
  std::partition (foo.begin(),foo.end(),IsOdd);

  // print contents again:
  std::cout << "foo:"; for (int& x:foo) std::cout << ' ' << x;
  if ( std::is_partitioned(foo.begin(),foo.end(),IsOdd) )
    std::cout << " (partitioned)\n";
  else
    std::cout << " (not partitioned)\n";

  return 0;
}

Possible output: 
foo: 1 2 3 4 5 6 7 (not partitioned)
foo: 1 7 3 5 4 6 2 (partitioned)


Complexity Up to linear in the distance between first and last: Calls pred for each element until a mismatch is found.

Data races Some (or all) of the objects in the range [first,last) are accessed (once at most).

Exceptions Throws if either pred or an operation on an iterator throws.
Note that invalid parameters cause undefined behavior.

See also
partition
Partition range in two (function template)
partition_point
Get partition point (function template)

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