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

function template

<functional>

template <class Arg, class Result>  pointer_to_unary_function<Arg,Result> ptr_fun (Result (*f)(Arg));template <class Arg1, class Arg2, class Result>  pointer_to_binary_function<Arg1,Arg2,Result> ptr_fun (Result (*f)(Arg1,Arg2));

Convert function pointer to function object

Returns a function object that encapsulates function

.

Function objects are objects whose class defines member function operator(). This member function allows the object to be used with the same syntax as a regular function call. Several standard algorithms and adaptors are designed to be used with function objects.

It is defined with the same behavior as:

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template <class Arg, class Result>
  pointer_to_unary_function<Arg,Result> ptr_fun (Result (*f)(Arg))
{
  return pointer_to_unary_function<Arg,Result>(f);
}

template <class Arg1, class Arg2, class Result>
  pointer_to_binary_function<Arg1,Arg2,Result> ptr_fun (Result (*f)(Arg1,Arg2))
{
  return pointer_to_binary_function<Arg1,Arg2,Result>(f);
}

Arg, Arg1, Arg2

Types of the function's arguments.

Result

Function's return type.

f

Pointer to a function, taking either one argument (of type Arg) or two arguments (of types Arg1 and Arg2) and returning a value of type Result.

A function object equivalent to

.

and

are function object types, derived respectively from

and

.

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// ptr_fun example
#include <iostream>
#include <functional>
#include <algorithm>
#include <cstdlib>
#include <numeric>
using namespace std;

int main () {
  char* foo[] = {"10","20","30","40","50"};
  int bar[5];
  int sum;
  transform (foo, foo+5, bar, ptr_fun(atoi) );
  sum = accumulate (bar, bar+5, 0);
  cout << "sum = " << sum << endl;
  return 0;
}

Possible output:

mem_fun

Convert member function to function object (pointer version) (function template)

pointer_to_unary_function

Generate unary function object class from pointer (class template)

pointer_to_binary_function

Generate binary function object class from pointer (class template)

unary_function

Unary function object base class (class template)

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