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### Descriptor service requirements

A descriptor service must meet the requirements for an I/O object service with support for movability, as well as the additional requirements listed below.

In the table below, `X` denotes a descriptor service class, `a` and `ao` denote values of type `X`, `b` and `c` denote values of type `X::implementation_type`, `n` denotes a value of type `X::native_handle_type`, `ec` denotes a value of type `error_code`, `i` denotes a value meeting `IoControlCommand` requirements, and `u` and `v` denote identifiers.

Table 9. DescriptorService requirements

expression | return type | assertion/note pre/post-condition
--- | --- | ---
`X::native_handle_type` |  | The implementation-defined native representation of a descriptor. Must satisfy the requirements of `CopyConstructible` types (C++ Std, 20.1.3), and the requirements of `Assignable` types (C++ Std, 23.1).
`a.construct(b);` |  | From IoObjectService requirements. post: `!a.is_open(b)`.
`a.destroy(b);` |  | From IoObjectService requirements. Implicitly cancels asynchronous operations, as if by calling `a.close(b, ec)`.
    a.move_construct(b, c);
|  | From IoObjectService requirements. The underlying native representation is moved from `c` to `b`.
    a.move_assign(b, ao, c);
|  | From IoObjectService requirements. Implicitly cancels asynchronous operations associated with `b`, as if by calling `a.close(b, ec)`. Then the underlying native representation is moved from `c` to `b`.
    a.assign(b, n, ec);
| `error_code` | pre: `!a.is_open(b)`. post: `!!ec || a.is_open(b)`.
    a.is_open(b);
| `bool` |
    const X& u = a;
    const X::implementation_type& v = b;
    u.is_open(v);
| `bool` |
    a.close(b, ec);
| `error_code` | If `a.is_open()` is true, causes any outstanding asynchronous operations to complete as soon as possible. Handlers for cancelled operations shall be passed the error code `error::operation_aborted`. post: `!a.is_open(b)`.
    a.native_handle(b);
| `X::native_handle_type` |
    a.cancel(b, ec);
| `error_code` | pre: `a.is_open(b)`. Causes any outstanding asynchronous operations to complete as soon as possible. Handlers for cancelled operations shall be passed the error code `error::operation_aborted`.
    a.io_control(b, i, ec);
| `error_code` | pre: `a.is_open(b)`.