Handle service requirements

A handle 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 handle 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, and u and v denote identifiers.

Table 14. HandleService requirements

expression	return type	assertion/note pre/post-condition
`X::native_handle_type`		The implementation-defined native representation of a handle. 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`.