- Title
- Query objects must be default constructible
- Status
- wp
- Section
- [exec.queryable.general]
- Submitter
- Eric Niebler
Created on 2025-05-07.00:00:00 last changed 1 month ago
Messages
Proposed resolution:
This wording is relative to N5014.
Modify [customization.point.object] as indicated:
-1- A customization point object is a function object ([function.objects]) with a literal class type that interacts with program-defined types while enforcing semantic requirements on that interaction.
-2- The type of a customization point object, ignoring cv-qualifiers, shall model `semiregular` ([concepts.object]) and shall be a structural type ([temp.param]) and a trivially copyable type ([class.prop]). Every constructor of this type shall have a non-throwing exception specification ([except.spec]).
[ Kona 2025-11-08; Status changed: Immediate → WP. ]
[ Kona 2025-11-07; approved by LWG. Status changed: New → Immediate. ]
[ 2025-11-05; Tim provides improved wording ]
LWG decided to guarantee some additional properties for CPOs.
[ 2025-10-23; Reflector poll. ]
Set priority to 2 after reflector poll.
"The discussion is wrong, `semiregular` requires `default_initializable`. If we want to mandate nothrow construction (a.k.a the implementation isn't out to get you), I'd rather we do it for all CPOs."
This wording is relative to N5008.
Modify [exec.queryable.general] as indicated:
-1- A queryable object is a read-only collection of key/value pair where each key is a customization point object known as a query object. The type of a query object satisfies `default_initializable`, and its default constructor is not potentially throwing. A query is an invocation of a query object with a queryable object as its first argument and a (possibly empty) set of additional arguments. A query imposes syntactic and semantic requirements on its invocations.
Imported from cplusplus/sender-receiver #333.
We require the types of query objects such as `get_scheduler` to be customization point objects. [customization.point.object] requires them to be `semiregular` but that concept does not require default constructability. Much of `std::execution` assumes query object types to be default constructible. I propose adding a (nothrow) default-constructibility requirement.
| History | |||
|---|---|---|---|
| Date | User | Action | Args |
| 2025-11-11 10:48:55 | admin | set | messages: + msg15667 |
| 2025-11-11 10:48:55 | admin | set | status: immediate -> wp |
| 2025-11-07 02:49:55 | admin | set | messages: + msg15579 |
| 2025-11-07 02:49:55 | admin | set | status: new -> immediate |
| 2025-11-06 03:53:16 | admin | set | messages: + msg15553 |
| 2025-10-23 12:39:14 | admin | set | messages: + msg15395 |
| 2025-05-18 09:48:20 | admin | set | messages: + msg14757 |
| 2025-05-07 00:00:00 | admin | create | |