What is the difference between nullptr and nullptr_t in C++?

Question

Which one should I use? Any advantages if I use one over the other?

Answer 1

nullptr is the constant, nullptr_t is its type. Use each one in contexts where you need respectively a null pointer, or the type of a null pointer.

Answer 2

nullptr is of type nullptr_t.

Answer 3

If you try this

cout << typeid(nullptr).name() << endl;

you will see that nullptr is of type std::nullptr_t.

Answer 4

nullptr is a pointer literal of type std::nullptr_t. And moreover nullptr is also a keyword of the C++ the same way as boolean literals false and true.:)

Answer 5

From [lex.nullptr]:

Pointer Literals

pointer-literal:
     nullptr

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 4.10 and 4.11. —end note ]

So use nullptr when you need a pointer literal, and std::nullptr_t in a context when you need to take that type. The latter, for instance, if you're making a function or constructor or something that can take a nullptr as an argument.

Answer 6

"... if I use one over the other?"

You can't (use one over the other) they're orthogonal by these means:

nullptr_t is the type used to represent a nullptr

nullptr is ⁽¹⁾effectively a constant of type nullptr_t that represents a specific compiler implementation defined value.

See the C++11 standards section:

2.14.7 Pointer literals

1. 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 4.10 and 4.11. — end note ]

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^{1) Just like the this keyword nullptr stands for an rvalue rather than being of const type. Thus, decltype(nullptr) can be a non-const type. With Visual C++ 2015 and MinGW g++ 5.1 it is non-const.}

Answer 7

In exactly the same way that true is a C++ keyword literal of type bool, nullptr is a C++ keyword literal of type std::nullptr_t.