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### 842. `ConstructibleAsElement` and bit containers

Section: 24.2 [container.requirements], 24.3.12 [vector.bool], 22.9.2 [template.bitset] Status: CD1 Submitter: Howard Hinnant Opened: 2008-06-03 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [container.requirements].

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Discussion:

24.2 [container.requirements] p. 3 says:

> Objects stored in these components shall be constructed using `construct_element` (20.6.9). For each operation that inserts an element of type `T` into a container (`insert`, `push_back`, `push_front`, `emplace`, etc.) with arguments `args... T` shall be `ConstructibleAsElement`, as described in table 88. [Note: If the component is instantiated with a scoped allocator of type `A` (i.e., an allocator for which `is_scoped_allocator<A>::value` is `true`), then `construct_element` may pass an inner allocator argument to `T`'s constructor. -- end note]

However `vector<bool, A>` (24.3.12 [vector.bool]) and `bitset<N>` (22.9.2 [template.bitset]) store bits, not `bool`s, and `bitset<N>` does not even have an allocator. But these containers are governed by this clause. Clearly this is not implementable.

Proposed resolution:

Change 24.2 [container.requirements] p. 3:

> Objects stored in these components shall be constructed using `construct_element` (20.6.9), unless otherwise specified. For each operation that inserts an element of type `T` into a container (`insert`, `push_back`, `push_front`, `emplace`, etc.) with arguments `args... T` shall be `ConstructibleAsElement`, as described in table 88. [Note: If the component is instantiated with a scoped allocator of type `A` (i.e., an allocator for which `is_scoped_allocator<A>::value` is `true`), then `construct_element` may pass an inner allocator argument to `T`'s constructor. -- end note]

Change 24.3.12 [vector.bool]/p2:

> Unless described below, all operations have the same requirements and semantics as the primary `vector` template, except that operations dealing with the `bool` value type map to bit values in the container storage, and `construct_element` (24.2 [container.requirements]) is not used to construct these values.

Move 22.9.2 [template.bitset] to clause 20.