1) Copies the elements from the range [first, last) to two different ranges depending on the value returned by the predicate p. The elements that satisfy the predicate p are copied to the range beginning at d_first_true. The rest of the elements are copied to the range beginning at d_first_false.

The behavior is undefined if the input range overlaps either of the output ranges.

unary predicate which returns ​true if the element should be placed in d_first_true.

The signature of the predicate function should be equivalent to the following:

bool pred(const Type &a);

The signature does not need to have const&, but the function must not modify the objects passed to it.
The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type.
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ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of ForwardIterator. ForwardIt1's value type must be CopyAssignable, writable to ForwardIt2 and ForwardIt3, and convertible to UnaryPredicate's argument type

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 three standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.