Section: 18.104.22.168 [meta.unary.prop] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:Addresses US 143
An is_aggregate type trait is needed. The emplace idiom is now common throughout the library, but typically relies on direct non-list initalization, which does not work for aggregates. With a suitable type trait, we could extend direct non-list-initlaization to perform aggregate-initalization on aggregate types.
Add a new row to Table 38:
template <class T> struct is_aggregate;
T is an aggregate type ([dcl.init.aggr])remove_all_extents_t<T> shall be a complete type, an array type, or (possibly cv-qualified) void.
[2017-02-26, Daniel comments and completes wording]
The proposed wording is incomplete, since it doesn't update the <type_traits> header synopsis, also ensuring that the corresponding is_aggregate_v variable template is defined.
[2017-03-01, Daniel comments and adjusts wording]
There is only one minor change performed, namely to mark the constant in the is_aggregate_v variable template as inline following the LWG preferences for "full inline" bullet list (B2) from P0607R0.
[2017-03-03, Kona, LEWG]
No interest in doing the is-list-initializable instead. No objection to getting that proposal also.Want this for C++17? SF | F | N | A | SA 2 | 8 | 1 | 3 | 0 Want this for C++20? SF | F | N | A | SA 6 | 5 | 2 | 0 | 0 Remember to correct the wording to: "remove_all_extents_t<T> shall be a complete type or cv-void."
[2017-03-03, Daniel updates wording]
In sync with existing writing, this was changed to "remove_all_extents_t<T> shall be a complete type or cv void"
Accepted as Immediate to resolve NB comment.
This will require a new complier intrinsic; Alisdair checked with Core and they're OK with that
This wording is relative to N4640.
Modify 23.15.2 [meta.type.synop], <type_traits> header synopsis, as indicated:
// 22.214.171.124, type properties […] template <class T> struct is_final; […] // 126.96.36.199, type properties […] template <class T> constexpr bool is_final_v = is_final<T>::value; […]
Add a new row to Table 42 — "Type property predicates":