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The type of the function's return value is determined by the [@boost:/doc/html/boost_asio/reference/async_result.html `net::async_result`] customization point, which comes with specializations for common library types such as `std::future` and may also be specialized for user-defined types. The body of the initiating function calls the [@boost:/doc/html/boost_asio/reference/async_initiate.html `net::async_initiate`] helper to capture the arguments and forward them to the specialization of `async_result`. An additional "initiation function" object is provided which `async_result` may use to immediately launch the operation, or defer the launch of the operation until some point in the future (this is called "lazy execution"). The initiation function object receives the internal completion handler which matches the signature expected by the initiating function:
The type of the function's return value is determined by the函数返回值类型由 [@boost:/doc/html/boost_asio/reference/async_result.html `net::async_result`] customization point, which comes with specializations for common library types such as `std::future` and may also be specialized for user-defined types. The body of the initiating function calls the [@boost:/doc/html/boost_asio/reference/async_initiate.html `net::async_initiate`] helper to capture the arguments and forward them to the specialization of `async_result`. An additional "initiation function" object is provided which `async_result` may use to immediately launch the operation, or defer the launch of the operation until some point in the future (this is called "lazy execution"). The initiation function object receives the internal completion handler which matches the signature expected by the initiating function:定制点决定。该定制点为 `std::future` 等常用库类型提供特化版本，同时也支持用户自定义类型的特化。发起函数的函数体通过调用 [@boost:/doc/html/boost_asio/reference/async_initiate.html `net::async_initiate`] 辅助函数来捕获参数并将其转发给 `async_result` 的特化版本。此外，还提供一个额外的“发起函数”对象，`async_result` 可利用该对象立即启动操作，或将操作启动延迟到未来某个时刻（即“延迟执行”）。该发起函数对象接收一个内部完成处理器，其签名与发起函数所期望的签名相匹配。
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Note that a `spawn` function itself has a completion signature, but we're ignoring it's result in the example by using `asio::detached`.
Note that a `spawn` function itself has a completion signature, but we're ignoring it's result in the example by using需要注意的是，`spawn` 函数本身也带有完成签名，但示例中通过使用 `asio::detached`. 忽略了它的结果。
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In both of these cases, an object with a specific type is used in place of the completion handler, and the return value of the initiating function is transformed from `void` to `std::future<std::size_t>` or `std::size_t`. The handler is sometimes called a __CompletionToken__ when used in this context. The return type transformation is supported by customization points in the initiating function signature. Here is the signature for [@boost:/doc/html/boost_asio/reference/async_write/overload1.html `net::async_write`]:
In both of these cases, an object with a specific type is used in place of the completion handler, and the return value of the initiating function is transformed from在这两种情况下，都使用具有特定类型的对象来替代完成处理器，且发起函数的返回值也从 `void` to转变为 `std::future<std::size_t>` or或 `std::size_t`. The handler is sometimes called a __CompletionToken__ when used in this context. The return type transformation is supported by customization points in the initiating function signature. Here is the signature for。在这种上下文中，该处理器有时被称为 __CompletionToken__。发起函数签名中的定制点支持这种返回值类型的转换。以下是 [@boost:/doc/html/boost_asio/reference/async_write/overload1.html `net::async_write`]: 的签名：
06/11/2026
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This functionality is enabled by passing the variable [@boost:/doc/html/boost_asio/reference/use_future.html `net::use_future`] (of type [@boost:/doc/html/boost_asio/reference/use_future_t.html `net::use_future_t<>`]) in place of the completion handler. The same `async_write` function overload can work with a [@https://en.wikipedia.org/wiki/Fiber_(computer_science) ['fiber]] launched with [@boost:/doc/html/boost_asio/reference/spawn/overload1.html `asio::spawn`]:
This functionality is enabled by passing the variable此功能通过将变量 [@boost:/doc/html/boost_asio/reference/use_future.html `net::use_future`] (of type（类型为 [@boost:/doc/html/boost_asio/reference/use_future_t.html `net::use_future_t<>`]) in place of the completion handler. The same `async_write` function overload can work with a [@https://en.wikipedia.org/wiki/Fiber_(computer_science) ['fiber]] launched with [@boost:/doc/html/boost_asio/reference/spawn/overload1.html `asio::spawn`]:）传递给完成处理器位置来实现。同一个 `async_write` 函数重载还可以与通过 [@boost:/doc/html/boost_asio/reference/spawn/overload1.html `asio::spawn`] 启动的 [@https://en.wikipedia.org/wiki/Fiber_(computer_science) [纤程]] 配合使用：
06/11/2026
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Because completion handlers cause an inversion of the flow of control, sometimes other methods of attaching a continuation are desired. Networking provides the [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3747.pdf ['Universal Model for Asynchronous Operations]], providing a customizable means for transforming the signature of the initiating function to use other types of objects and methods in place of a completion handler callback. For example to call to write a string to a socket asynchronously, using a `std::future` to receive the number of bytes transferred thus looks like this:
Because completion handlers cause an inversion of the flow of control, sometimes other methods of attaching a continuation are desired. Networking provides the [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3747.pdf ['Universal Model for Asynchronous Operations]], providing a customizable means for transforming the signature of the initiating function to use other types of objects and methods in place of a completion handler callback. For example to call to write a string to a socket asynchronously, using a `std::future` to receive the number of bytes transferred thus looks like this:由于完成处理器会导致控制流反转，有时需要其他方式来附加延续。网络库提供 [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3747.pdf [异步操作通用模型]]，该模型提供一种可定制的方法，用于改变发起函数的签名，以便使用其他类型的对象和方法来替代完成处理器回调。例如，使用 `std::future` 接收传输字节数来异步向套接字写入字符串的调用方式如下：
06/11/2026
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Universal Model
Universal Model通用模型
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I/O objects such as sockets and streams [*are not thread-safe]. Although it is possible to have more than one operation outstanding (for example, a simultaneous asynchronous read and asynchronous write) the stream object itself may only be accessed from one thread at a time. This means that member functions such as move constructors, destructors, or initiating functions must not be called concurrently. Usually this is accomplished with synchronization primitives such as a [@https://en.cppreference.com/w/cpp/thread/mutex `mutex`], but concurrent network programs need a better way to access shared resources, since acquiring ownership of a mutex could block threads from performing uncontended work. For efficiency, networking adopts a model of using threads without explicit locking by requiring all access to I/O objects to be performed within a [@boost:/doc/html/boost_asio/overview/core/strands.html ['strand]].
I/O objects such as sockets and streams [*are not thread-safe]. Although it is possible to have more than one operation outstanding (for example, a simultaneous asynchronous read and asynchronous write) the stream object itself may only be accessed from one thread at a time. This means that member functions such as move constructors, destructors, or initiating functions must not be called concurrently. Usually this is accomplished with synchronization primitives such as a [@https://en.cppreference.com/w/cpp/thread/mutex `mutex`], but concurrent network programs need a better way to access shared resources, since acquiring ownership of a mutex could block threads from performing uncontended work. For efficiency, networking adopts a model of using threads without explicit locking by requiring all access to I/O objects to be performed within a套接字和流等 I/O 对象 [*不是线程安全的]。尽管可以同时有多个操作未完成（例如，同时进行异步读和异步写），但流对象本身在同一时刻只能从一个线程进行访问。这意味着移动构造函数、析构函数或发起函数等成员函数不能并发调用。通常，这可以通过互斥锁等同步原语来实现，但并发网络程序需要更好的方式来访问共享资源，因为获取互斥锁的所有权可能会阻塞线程，使其无法执行无竞争的工作。为了提高效率，网络库采用了一种无需显式加锁的线程使用模型，要求所有对 I/O 对象的访问都必须在一个 [@boost:/doc/html/boost_asio/overview/core/strands.html ['strand]].串行器]] 内执行。
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Concurrency
Concurrency并发
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The __AsyncReadStream__ and __AsyncWriteStream__ concepts define requirements for ['asynchronous streams]: a portable I/O abstraction that exchanges data asynchronously using buffer sequences to represent bytes and `error_code` to report any failures. An ['asynchronous stream algorithm] is written as a templated initiating function template accepting a stream object meeting the named requirements for asynchronous reading, writing, or both. This example shows an algorithm which writes some text to an asynchronous stream:
The __AsyncReadStream__ and与 __AsyncWriteStream__ concepts define requirements for ['asynchronous streams]: a portable I/O abstraction that exchanges data asynchronously using buffer sequences to represent bytes and `error_code` to report any failures. An ['asynchronous stream algorithm] is written as a templated initiating function template accepting a stream object meeting the named requirements for asynchronous reading, writing, or both. This example shows an algorithm which writes some text to an asynchronous stream:概念用于定义 [异步流] 的要求：这是一种可移植的 I/O 抽象，能够异步交换数据，使用缓冲区序列表示字节数据，并通过 `error_code` 报告错误。[异步流算法] 以模板化的发起函数形式编写，接受满足异步读、异步写或两者兼具命名要求的流对象。以下示例用于展示一个向异步流写入文本的算法：
06/11/2026
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Networking prescribes facilities to determine the context in which handlers run. Every I/O object refers to an __ExecutionContext__ for obtaining the __Executor__ instance used to invoke completion handlers. An executor determines where and how completion handlers are invoked. Executors obtained from an instance of __io_context__ offer a basic guarantee: handlers will only be invoked from threads which are currently calling [@boost:/doc/html/boost_asio/reference/io_context/run/overload1.html `net::io_context::run`].
Networking prescribes facilities to determine the context in which handlers run. Every I/O object refers to an __ExecutionContext__ for obtaining the __Executor__ instance used to invoke completion handlers. An executor determines where and how completion handlers are invoked. Executors obtained from an instance of __io_context__ offer a basic guarantee: handlers will only be invoked from threads which are currently calling网络库规定用于确定处理器运行上下文的设施。每个 I/O 对象都关联一个 __ExecutionContext__，用于获取调用完成处理器所需的 __Executor__ 实例。执行器决定完成处理器的调用位置与方式。从 __io_context__ 实例获取的执行器提供一项基本保证：处理器仅会在当前正在调用 [@boost:/doc/html/boost_asio/reference/io_context/run/overload1.html `net::io_context::run`]. 的线程中被执行。
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# `total`
# `total` —— 完全型
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# `partial`
# `partial` —— 部分型
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# `terminal`
# `terminal` —— 终止型`
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# `terminal`
# `terminal终止型`
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# `terminal`
# `terminal`
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The available [@boost:/doc/html/boost_asio/reference/cancellation_type.html cancellation types] are listed below.
The available可用的 [@boost:/doc/html/boost_asio/reference/cancellation_type.html cancellation types] are listed below.取消类型] 如下所列。
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A completion token's associated cancellation_slot can be used to cancel single operations. This is often passed through by the completion token such as [@boost:/doc/html/boost_asio/reference/use_awaitable.html `net::use_awaitable`] or [@boost:/doc/html/boost_asio/reference/yield_context.html `net::yield_context`] .
A completion token's associated cancellation_slot can be used to cancel single operations. This is often passed through by the completion token such as完成处理器令牌所关联的取消槽可用于取消单个操作。该取消槽通常由完成处理器令牌（例如 [@boost:/doc/html/boost_asio/reference/use_awaitable.html `net::use_awaitable`] or或 [@boost:/doc/html/boost_asio/reference/yield_context.html `net::yield_context`] .）进行传递。
06/11/2026
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The function [@boost:/doc/html/boost_asio/reference/bind_allocator.html `net::bind_allocator`] can be used when the caller wants to assign a custom allocator to the operation.
The function当调用方希望为操作指定自定义分配器时，可以使用 [@boost:/doc/html/boost_asio/reference/bind_allocator.html `net::bind_allocator`] can be used when the caller wants to assign a custom allocator to the operation.函数。
06/11/2026
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The allocator is used by the implementation to obtain any temporary storage necessary to perform the operation. Temporary allocations are always freed before the completion handler is invoked. The executor is a cheaply copyable object providing the algorithm used to invoke the completion handler. Unless customized by the caller, a completion handler defaults to using `std::allocator<void>` and the executor of the corresponding I/O object.
The allocator is used by the implementation to obtain any temporary storage necessary to perform the operation. Temporary allocations are always freed before the completion handler is invoked. The executor is a cheaply copyable object providing the algorithm used to invoke the completion handler. Unless customized by the caller, a completion handler defaults to using `std::allocator<void>` and the executor of the corresponding I/O object.实现使用该分配器来获取执行操作所需的任何临时存储空间。所有临时分配的内存在完成处理器被调用之前都会被释放。执行器是一种轻量可复制的对象，提供用于调用完成处理器的算法。除非调用方进行了自定义，否则完成处理器默认使用 `std::allocator<void>` 以及对应 I/O 对象的执行器。
06/11/2026

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