为什么是“std::is_pointer<std::nullptr_t> ::value" 等于 false？</std::nullptr_t>

Question

I read about std::is_pointer in C++.

Then I wrote the program and check whether T is a pointer type or not using std::is_pointer .

#include <iostream>

int main() 
{
    std::cout<<std::boolalpha;
    std::cout<<"char : " <<std::is_pointer<char>::value<<std::endl; // OK
    std::cout<<"char * : "<<std::is_pointer<char *>::value<<std::endl; // OK
    std::cout<<"char ** : "<<std::is_pointer<char **>::value<<std::endl; // OK
    std::cout<<"char *** : "<<std::is_pointer<char ***>::value<<std::endl; // OK
    std::cout<<"std::nullptr_t : "<<std::is_pointer<std::nullptr_t>::value<<std::endl;  // Not ok, Why false??
}

Output : [Wandbox Demo]

char : false
char * : true
char ** : true
char *** : true
std::nullptr_t : false

Why is std::is_pointer<std::nullptr_t>::value equal to false ?

Answer 1

Because std::nullptr_t is not a pointer type . And is_pointer only evaluates to true for pointer types.

nullptr_t is convertible to a pointer, but it isn't a pointer. Indeed, nullptr_t is not a class type, integral, floating-point, enumerator, or any kind of type other than is_null_pointer type. It has its own unique classification in the categorization of types.

Answer 2

Because it's not a pointer type. It's a distinct type that is implicitly convertible to any pointer type.

As the note in [lex.nullptr] summarizes:

The pointer literal is the keyword nullptr. It is a prvalue of type std​::​nullptr_t. [ Note: std​::​nullptr_t is a distinct type that is neither a pointer type nor a pointer to member type; rather, a prvalue of this type is a null pointer constant and can be converted to a null pointer value or null member pointer value. See [conv.ptr] and [conv.mem]. — end note ]

Answer 3

As unintuitive as it sounds, std::nullptr_t is not a pointer type. For instance, you cannot dereference a std::nullptr_t , so it'd be pretty weird if is_pointer_type<nullptr_t>::value was true .

It is merely convertible to any pointer type.

Answer 4

Because std::nullptr_t is not a pointer type itself, but the type of the null pointer literal nullptr .

std::nullptr_t is the type of the null pointer literal, nullptr . It is a distinct type that is not itself a pointer type or a pointer to member type.

From the standard, 21.2.3 Null pointers [support.types.nullptr]

The type nullptr_t is a synonym for the type of a nullptr expression, and it has the characteristics described in [basic.fundamental] and [conv.ptr].

Answer 5

It's definitely unintuitive in some cases, but it makes sense:

1. Value-wise, it doesn't behave like a pointer; it cannot point to anything .
   For example, you cannot do

   T *p =...; reinterpret_cast<T *>(reinterpret_cast<std::nullptr_t>(p));

   and expect to get a pointer to the same address that p points to.

2. Type-wise, it doesn't behave like a pointer. For example,

    std::is_pointer<P>::value

   should imply

   std::is_same<P, std::remove_pointer<P>::type *>::value

   but that clearly isn't the case.