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Showing content from https://cplusplus.com/reference/functional/pointer_to_unary_function/ below:

class template

std::pointer_to_unary_function

template <class Arg, class Result> class pointer_to_unary_function;

Generate unary function object class from pointer

Generates an unary function object class from a pointer to a function that takes a single argument of type

Arg

and returns a value of type

Result

pointer_to_unary_function is generally used as a type. The function ptr_fun (also defined in header <functional>) can be used to directly construct an object of this type.

This class is derived from unary_function and is typically defined as:

template <class Arg, class Result>
  class pointer_to_unary_function : public unary_function <Arg,Result>
{
protected:
  Result(*pfunc)(Arg);
public:
  explicit pointer_to_unary_function ( Result (*f)(Arg) ) : pfunc (f) {}
  Result operator() (Arg x) const
    { return pfunc(x); }
};

Members

constructor: Constructs an unary function object class from pointer to a function that takes one argument of type Arg and returns a value of type Result.
operator(): Member function taking a single parameter and returning the result of calling the function pointer used at construction.

Example

// pointer_to_unary_function example
#include <iostream>
#include <functional>
#include <algorithm>
#include <cmath>
using namespace std;

int main () {
  pointer_to_unary_function <double,double> LogObject (log);
  double numbers[] = {10.0, 20.0, 40.0, 80.0, 160.0};
  double logs[5];
  transform (numbers, numbers+5, logs, LogObject);
  for (int i=0; i<5; i++)
    cout << logs[i] << " ";
  cout << endl;
  return 0;
}

Possible output:

2.30259 2.99573 3.68888 4.38203 5.07517