What is the null pointer in C++?

#include <iostream>
using namespace std;
struct MyStruct { };
void pointer_to_fundamental_type(int* ptr) {
    std::cout << "Hello from void pointer_to_fundamental_type(int*)\n";
}
void pointer_to_user_defined_type(MyStruct* ptr) {
    std::cout << "Hello from void pointer_to_user_defined_type(MyStruct*)\n";
}
// Accepts pointer to member of MyStruct that has
// int type
void pointer_to_member_object(int MyStruct::*) {
  std::cout << "Hello from void pointer_to_member_object(int MyStruct::*)\n";
}
// Accepts pointer to member function of MyStruct
// that has int return type and no arguments
void pointer_to_member_function(int (MyStruct::*)(void)) {
  std::cout << "Hello from pointer_to_member_function(int (MyStruct::*)(void))\n";
}
int main() {
  std::nullptr_t null = nullptr;
  // nullptr is convertible to null pointer of
  // any pointer type:
  pointer_to_fundamental_type(nullptr);
  pointer_to_user_defined_type(null);
  // and any pointer to member type:
  pointer_to_member_object(nullptr);
  pointer_to_member_function(null);
}

#include <cstddef> // contains NULL pointer constant definition
#include <iostream>
template<class T>
constexpr T clone(const T& t)
{
    return t; // Return copy of t
}
 
void g(int*)
{
    std::cout << "Function g called\n";
}
 
int main()
{
    g(nullptr);        // Fine
    g(NULL);           // Fine
    g(0);              // Fine
 
    g(clone(nullptr)); // Fine
//  g(clone(NULL));    // ERROR: invalid conversion from ‘long int’ to ‘int*’ 
//  g(clone(0));       // ERROR: invalid conversion from ‘int’ to ‘int*’ 
    int* ptr = nullptr; // Fine
    int* null_ptr = NULL; // Fine
    int* zero_ptr = 0; // Fine
    int* clone_ptr = clone(nullptr); // Fine
//  int* clone_null_ptr = clone(NULL); // error: error: invalid conversion from ‘long int’ to ‘int*’ 
//  int* clone_zero_ptr = clone(0); // error: invalid conversion from `int` to `int*`
}