Что использовать с указателями? 0, NULL, nullptr?

#include <iostream>
#include <cstdlib>
 
void foo(const int&)
{
    std::cout << "foo(const int&)" << std::endl;
}
 
void foo(const int*)
{
    std::cout << "foo(const int*)" << std::endl;
}
 
int main()
{
    foo(NULL);
    foo(nullptr);
    return 0;
}

namespace std {
typedef decltype(nullptr) nullptr_t;
}

if (!pointer) {}
if (pointer!=nullptr){} // аналогично первому

static struct nullptr_t {template<typename T> operator T* () const {return 0;}} nullptr;

static struct nullptr_t {
    template<typename T> operator T* () const {return 0;}
    operator bool () const {return false;}
} nullptr;

const short NULL = 0;

const short NULL = 0;

#ifdef NULL
#undef NULL
const short NULL = 0;

#include <iostream>
#include <cassert>
 
const int size = 5;
 
#define size 6
 
int main()
{
    assert(size == 5);
}

#ifndef HEADER_H
#define HEADER_H
 
class foo
{
private:
    int private_field;
};
 
#endif // HEADER_H

#include <iostream>
 
#define private public
#include "header.h"
 
int main()
{
    foo f;
    f.private_field = 5;
    std::cout << f.private_field << std::endl;
}