This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of New status.

### 3484. Should `<stddef.h>` declare `::nullptr_t`?

Section: 17.15.7 [support.c.headers.other] Status: New Submitter: Thomas Köppe Opened: 2020-09-06 Last modified: 2022-09-18

Priority: 3

View other active issues in [support.c.headers.other].

View all other issues in [support.c.headers.other].

View all issues with New status.

Discussion:

From this editorial issue request:

The header `<stddef.h>` is currently specified in 17.15.7 [support.c.headers.other] to not declare a global name corresponding to `std::byte`, but no similar exclusion exists for `std::nullptr_t`.

Is an oversight or intentional? There does not seem to be an interoperability reason to provide a global namespace name `::nullptr_t`, since this construction would be meaningless in C and thus the name would not be encountered in code that is both valid C and C++.

For lack of justification, I would like to propose to require normatively that no name `::nullptr_t` be declared by `<stddef.h>`.

Additional notes: The proposing paper N2431 mentions only an addition of "`nullptr_t`" to `<cstddef>` and does not discuss the impact on `<stddef.h>`. By omission this means the default rules for `<stddef.h>` apply and the global name should exist, but this does not provide us with a positive signal of intention.

I also realize that this is a rather obscure point, and that vendors are already shipping `::nullptr_t`, so I am also happy to drop this issue as not being worth the churn and the increase in implementation complexity (since removals don't generally simplify implementations). I would welcome a bit of discussion, though.

[2020-09-29; Priority to P3 after reflector discussions]

[2022-09-18; Daniel comments]

See also 3782, which points out that `nullptr_t` has become a part of the C standard library as of C23 (see WG14-N3042 and WG14-N3048).

Proposed resolution:

This wording is relative to N4910.

1. Modify 17.15.7 [support.c.headers.other] as indicated:

   > -1- Every C header other than `<complex.h>` (17.15.2 [complex.h.syn]), `<iso646.h>` (17.15.3 [iso646.h.syn]), `<stdalign.h>` (17.15.4 [stdalign.h.syn]), `<stdatomic.h>` (33.5.12 [stdatomic.h.syn]), `<stdbool.h>` (17.15.5 [stdbool.h.syn]), and `<tgmath.h>` (17.15.6 [tgmath.h.syn]), each of which has a name of the form `<name.h>`, behaves as if each name placed in the standard library namespace by the corresponding `<cname>` header is placed within the global namespace scope, except for the functions described in 28.7.6 [sf.cmath], the declaration of `std::byte` and `std::nullptr_t` (17.2.1 [cstddef.syn]), and the functions and function templates described in 17.2.5 [support.types.byteops]. It is unspecified whether these names are first declared or defined within namespace scope (6.4.6 [basic.scope.namespace]) of the namespace `std` and are then injected into the global namespace scope by explicit using-declarations (9.9 [namespace.udecl]).

2. In [diff.cpp??] [Editorial note: new compatibility section to be created for C++20], add a new entry:

   > Affected subclause: 17.15.7 [support.c.headers.other]
   > Change: Removal of name `::nullptr_t` from header `<stddef.h>`.
   > Rationale: `std::nullptr_t` is a C++-only feature that is not useful for interoperability with ISO C.
   > Effect on original feature: Valid C++20 code may fail to compile in this International Standard. Occurrences of `#include <stddef.h>` and `nullptr_t` should be replaced with `#include <cstddef>` and `std::nullptr_t`.