Can nullptr be used as a variable argument (varargs)?

Question

Can I use the nullptr keyword as an argument for a variable function? If so, does it undergo any kind of standard conversion, and what is the type of the resulting value?

Concretely, is the following correct?

std::printf("%p", nullptr);

Or does it have to be:

std::printf("%p", static_cast<void *>(nullptr));

`nullptr` has the type of `nullptr_t`, no? Any thoughts about conversions should start there. — Bartek Banachewicz, Sep 11 '13 at 13:20
@BartekBanachewicz: I know, but I couldn't find a rule relating that type to the ellipsis. I was looking under standard conversions, and never checked the section on function calls. — Kerrek SB, Sep 11 '13 at 13:26

score 22 · Accepted Answer · edited Sep 11 '13 at 13:26

22

§5.2.2p7 When there is no parameter for a given argument, the argument is passed in such a way that the receiving function can obtain the value of the argument by invoking va_arg (18.10)... An argument that has (possibly cv-qualified) type std::nullptr_t is converted to type void* (4.10)...

edited Sep 11 '13 at 13:26

Bartek Banachewicz

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answered Sep 11 '13 at 13:23

Igor Tandetnik

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1

`+1`, my Standard-fu got a little rusty. – Bartek Banachewicz Sep 11 '13 at 13:27
4

I *finally* found it and then came back to 6 answers :p – chris Sep 11 '13 at 13:28

score 11 · Answer 2 · answered Sep 11 '13 at 13:22

The standard says that any argument of type nullptr_t will be converted to void* when matching .... So the call is correct without the cast.

EDIT:

From the standard (§5.2.2/7):

When there is no parameter for a given argument, the argument is passed in such a way that the receiving function can obtain the value of the argument by invoking va_arg. The lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the argument expression. An argument that has (possibly cv-qualified) type std::nullptr_t is converted to type void*.

Can nullptr be used as a variable argument (varargs)?

2 Answers2