Can the NULL macro actually be a nullptr?

Question

According to the draft of the standard N4713 (7.11/1):

A null pointer constant is an integer literal (5.13.2) with value zero or a prvalue of type std::nullptr_t.

and 21.2.3/2:

The macro NULL is an implementation-defined null pointer constant.

follow that NULL can be defined as nullptr. Same is mentioned on cppreference:

#define NULL 0 //since C++11 #define NULL nullptr 

At the same time "Additive operators" clause says (8.5.6/7):

If the value 0 is added to or subtracted from a null pointer value, the result is a null pointer value. If two null pointer values are subtracted, the result compares equal to the value 0 converted to the type std::ptrdiff_t.

Hence the following code should be valid:

0 + nullptr;  nullptr - nullptr;  

but because of the lack of +/- operators for std::nullptr_t the code is invalid.

Is there something that I didn't take into account or NULL macro can't be actually defined as nullptr?

Answer 1

While nullptr is a null pointer constant, it is not a null pointer value. The latter is a value of some pointer type, which std::nullptr_t is not.

Reference:

A null pointer constant is an integer literal (5.13.2) with value zero or a prvalue of type std::nullptr_t. A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type and is distinguishable from every other value of object pointer or function pointer type. Such a conversion is called a null pointer conversion. [...]

7.11/1 in N4659, emphasize mine

So NULL can indeed be nullptr without providing the arithmetic operators.

Answer 2

nullptr is a null pointer literal, and although the result of converting nullptr to a pointer type is the null pointer value, nullptr itself isn't of a pointer type, but of type std::nullptr_t. The arithmetic works if you do convert the nullptr to a pointer type:

0 + (int*)nullptr;  (int*)nullptr - (int*)nullptr; 

---

Can the NULL macro actually be a nullptr?

Yes, because nullptr is a null pointer literal.

Note that prior to C++11, the all of the null pointer literals in C++ happened to also be integer literals, so this bad code: char c = NULL; used to work in practice. If NULL is defined as nullptr, that code no longer works.