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### 2358. Apparently-bogus definition of `is_empty` type trait

Section: 20.15.5.4 [meta.unary.prop] Status: Open Submitter: Richard Smith Opened: 2014-02-01 Last modified: 2017-02-02

Priority: 3

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Discussion:

The 'Condition' for `std::is_empty` is listed as:

> "`T` is a class type, but not a union type, with no non-static data members other than bit-fields of length 0, no virtual member functions, no virtual base classes, and no base class `B` for which `is_empty<B>::value` is false."

This is incorrect: there is no such thing as a non-static data member that is a bit-field of length 0, since bit-fields of length 0 must be unnamed, and unnamed bit-fields are not members (see 11.4.10 [class.bit] p2).

It also means that classes such as:

>     struct S {
>      int : 3;
>     };

are empty (because they have no non-static data members). There's implementation divergence on the value of `is_empty<S>::value`.

I'm not sure what the purpose of `is_empty` is (or how it could be useful), but if it's desirable for the above type to not be treated as empty, something like this could work:

> "`T` is a class type, but not a union type, with no non-static data members other than, no unnamed bit-fields of non-zero length 0, no virtual member functions, no virtual base classes, and no base class `B` for which `is_empty<B>::value` is false."

and if the above type should be treated as empty, then this might be appropriate:

> "`T` is a class type, but not a union type, with no (named) non-static data members other than bit-fields of length 0, no virtual member functions, no virtual base classes, and no base class `B` for which `is_empty<B>::value` is false."

[2016-08 Chicago]

Walter says: We want `is_empty_v<S>` to produce false as a result. Therefore, we recommend adoption of the first of the issue's suggestions.

Tuesday AM: Moved to Tentatively Ready

Previous resolution [SUPERSEDED]:

[2016-10 by Marshall - this PR incorrectly highlighted changed portions]

> Modify Table 38 — Type property predicates for `is_empty` as follows:

> > `T` is a non-union class type with no non-static data members other than, no unnamed bit-fields of non-zero length 0, no virtual member functions, no virtual base classes, and no base class `B` for which `is_empty_v<B>` is false.

[2016-10 Telecon]

Should probably point at section 1.8 for some of this. Status back to 'Open'

Proposed resolution:

Modify Table 38 — Type property predicates for `is_empty` as follows:

> `T` is a class type, but not a union type,is a non-union class type with no non-static data members other than, no unnamed bit-fields of non-zero length 0, no virtual member functions, no virtual base classes, and no base class `B` for which `is_empty_v<B>` is false.