The relationship between 0 and null and nullptr in c++11 __c++

1. In essence 1) 0 is the literal value of int type constant 2) NULL is a preprocessing variable, defined in Cstdlib, its value is 0 3) nullptr is an nullptr_t type. The definition of NULL in 2.cstdlib #ifdef __cplusplus #define NULL 0 #else #define NULL ((void *) 0) #endif The reason for this definition is because in the C language
, allowing the void* type to be implicitly convertible to any pointer type, which is not allowed in C + +, but can assign a value of 0 to any type of pointer, so that NULL is defined as 0 in C + +. 
3.0 compared to NULL in the online data, all agree that 0 is better than NULL, because many may be wrong, using 0 more intuitive, such as: void Test (int i) {cout << "in int" << Endl;} void Test (int * i) {cout << "in int *" << Endl} int main () {test (0);//output in int test (NULL);//Output  in int} at this point, two function calls will call test with the parameter int, and the version using the 0 transfer looks more intuitive because 0 itself is a literal constant of type int, and subconsciously thinks NULL should be a pointer type, which is bad for error 4.nullptr than 0. Better nullptr is a nullptr_t type of constant that defines a conversion operator that is going to any pointer type, and does not allow an object of that type to be converted to a pointer type, and in the invocation example above, using nullptr, the second function is called, and the output "in int*", because Nullptr is converted to int *, which is consistent with our usual understanding. The types of nullptr_t can be defined as follows: 5.nullptr_t implementation class nullptr_t {Public:template<class t> inline operator () const//definition Type
        Conversion operator, so that nullptr_t can be converted to any non-class member pointer type {return 0; }//Overload type conversionsoperator so that nullptr_t can be converted to any pointer type in class C (data member pointer/function member pointer)//pointer to a data member in the class, T will be inferred to be the type of the data member Eg:int (X::*); T is int,c x//A pointer to a function member in a class, T is deduced to be the type of the function member Eg:int (X::*) (int); 			
At this point T is equivalent to: typedef int (T) (int) Template<class C, class t> inline operator T C::* () const {return 0;}
Private:void operator& () const;
}; Const NULL_PTR nullptr = {} 6.nullptr How to use the NULLPTR keyword to identify a null pointer, is a std::nullptr_t type (constexpr) variable.
It can be converted to any pointer type and bool Boolean type (primarily to accommodate the normal pointer as a conditional judgment statement), but cannot be converted to an integer.     char *p1 = nullptr;     correct int *p2 = nullptr;       correct bool B = nullptr; That's right.        if (b) is judged to be false int a = nullptr;
    ERROR 7. Test nullptr Union U {Long I;
nullptr_t T;

};
    int main () {u u;
    U.I = 3; printf ("%ld\n", (long) u.t); What it is?
0 or 3? So this is supposed to be in union semantics or nullptr semantics. This is not said in the standard, we have argued for a very long time. Of course, the implementation of our compiler still keeps the semantics of nullptr, and the result is 0.