This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-10-26

2530. Multiple definitions of enumerators

Section: 6.3  [basic.def.odr]     Status: C++23     Submitter: Naiver Miigon     Date: 2022-02-10

[Accepted as a DR at the February, 2023 meeting.]

Issue 2494 specified a list of definable items and required that no translation unit contain more than one definition of any of those items. However, the list omits enumeration constants, implicitly allowing an example like:

  enum E { e, e };

According to 6.1 [basic.pre] paragraph 3, an enumerator is an entity. According to 6.2 [basic.def] paragraph 2,

Each entity declared by a declaration is also defined by that declaration unless: ...

and enumerators are not on the list of excluded cases, so an enumerator-definition is a definition. Furthermore, 6.6 [basic.link] paragraph 8 says,

Two declarations of entities declare the same entity if, considering declarations of unnamed types to introduce their names for linkage purposes, if any (9.2.4 [dcl.typedef], 9.7.1 [dcl.enum]), they correspond (6.4.1 [basic.scope.scope]), have the same target scope that is not a function or template parameter scope, and either

In the example above, both enumerators thus define the same entity, so the one-definition rule is responsible for excluding the duplicate definitions but does not do so.

Suggested resolution [SUPERSEDED]:

Change 6.3 [basic.def.odr] paragraph 1 as follows:
Each of the following is termed a definable item:

Proposed resolution (approved by CWG 2022-12-02):

Change in 9.7.1 [dcl.enum] paragraph 2 as follows:

... The identifiers in an enumerator-list are declared as constants, and can appear wherever constants are required. The same identifier shall not appear as the name of multiple enumerators in an enumerator-list. An enumerator-definition with = gives the associated enumerator the value indicated by the constant-expression. If the first enumerator has no initializer, the value of the corresponding constant is zero. An enumerator-definition without an initializer gives the enumerator the value obtained by increasing the value of the previous enumerator by one.