Exit Zen
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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`]:
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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`.
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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:
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This transformed, internal handler is responsible for the finalizing step that delivers the result of the operation to the caller. For example, when using `net::use_future` the internal handler will deliver the result by calling [@https://en.cppreference.com/w/cpp/thread/promise/set_value `std::promise::set_value`] on the promise object returned by the initiating function.
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Using Networking
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Most library stream algorithms require a __socket__, __ssl_stream__, or other __Stream__ object that has already established communication with a remote peer. This example is provided as a reminder of how to work with sockets:
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Throughout this documentation identifiers with the following names have special meaning:
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Global Variables
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Name
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Description
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[@boost:/doc/html/boost_asio/reference/io_context.html [*`ioc`]]
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A variable of type __io_context__ which is running on one separate thread, and upon which an __executor_work_guard__ object has been constructed.
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[@boost:/doc/html/boost_asio/reference/ip__tcp/socket.html [*`sock`]]
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string ID 383
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A variable of type [@boost:/doc/html/boost_asio/reference/ip__tcp/socket.html `tcp::socket`] which has already been connected to a remote host.
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[@boost:/doc/html/boost_asio/reference/ssl__stream.html [*`ssl_sock`]]
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string ID 383
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A variable of type [@boost:/doc/html/boost_asio/reference/ssl__stream.html `net::ssl::stream<tcp::socket>`] which is already connected and has handshaked with a remote host.
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[link beast.ref.boost__beast__websocket__stream [*`ws`]]
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A variable of type [link beast.ref.boost__beast__websocket__stream `websocket::stream<tcp::socket>`] which is already connected with a remote host.
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