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?

`nullptr` and `nullptr_t` are never interchangeable. The question makes no sense. There's no such matter as choosing which one to use. — AnT stands with Russia, Jun 24 '15 at 20:23
There was a [very good answer](http://qr.ae/74om57) on the same question submitted on Quora earlier today by Brain. Not sure if you're the one who asked it as it seems very coincidental. — David G, Jun 24 '15 at 20:25

score 36 · Accepted Answer · answered Jun 24 '15 at 20:18

36

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.

answered Jun 24 '15 at 20:18

Quentin

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score 9 · Answer 2 · edited Jun 24 '15 at 20:39

"... 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

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 ]

^{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.}

score 7 · Answer 3 · answered Jun 24 '15 at 20:23

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.

answered Jun 24 '15 at 20:23

Julian McKinlay

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score 5 · Answer 4 · answered Jun 24 '15 at 20:18

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nullptr is of type nullptr_t.

answered Jun 24 '15 at 20:18

Sam Estep

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score 3 · Answer 5 · answered Jun 24 '15 at 20:18

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If you try this

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

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

answered Jun 24 '15 at 20:18

Karlis Olte

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score 3 · Answer 6 · answered Jun 24 '15 at 20:19

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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.:)

answered Jun 24 '15 at 20:19

Vlad from Moscow

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Barry · Answer 7 · 2015-06-24T20:34:46.963

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.

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

7 Answers7

2.14.7 Pointer literals