问将cctype函数赋值给std:: function，gcc编译错误

EN

#include <cstddef>
#include <cstdio>
#include <cstdint>

#include <string>
#include <tuple>
#include <functional>
#include <array>
#include <cctype>
//#include <ctype.h>

namespace abc
{
  int two_times( int a )
  {
    printf( "two_times being called\n" );
    return 2*a;
  }

  int three_times( int a )
  {
    printf( "three_times being called\n" );
    return 3*a;
  }
}

int main()
{
  typedef std::tuple< std::string, std::function<int(int)> > item_t;
  std::array< item_t, 3 > array{
    std::make_tuple( "three_times", abc::three_times ),
    std::make_tuple( "two_times", abc::two_times ),
    std::make_tuple( "iscntrl", std::iscntrl ) };

  int i = 2;

  for( const item_t& item : array )
  {
    printf( "operation: %s\n", std::get<0>(item).c_str() );
    int val = std::get<1>(item)( i );
    printf( "val: %d\n", val );
    putchar( '\n' );
  }

  getchar();
  return 0;
}

error: too many arguments to function 'constexpr std::tuple<typename std::__decay_and_strip<_Elements>::__type ...> std::make_tuple(_Elements&& ...) [with _Elements = {}]'
     std::make_tuple( "iscntrl", std::iscntrl ) };

std::function<int(int)> f;
f = abc::two_times;
printf( “result: %d\n“, f( 4 ) ); // so far fine
f = std::iscntrl;

error: no match for 'operator=' (operand types are 'std::function<int(int)>' and '<unresolved overloaded function type>')
   f = std::iscntrl;