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* Review the stream abstraction. This is the type of object, such as
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HTTP Comparison to Other Libraries
HTTP 与其他库的比较
There are a few C++ published libraries which implement some of the HTTP protocol. We analyze the message model chosen by those libraries and discuss the advantages and disadvantages relative to Beast.
目前已有一些 C++ 库实现了部分 HTTP 协议。我们将分析这些库所采用的消息模型，并讨论它们相对于 Beast 的优缺点。
The general strategy used by the author to evaluate external libraries is as follows:
作者评估外部库的总体策略如下：
* Review the message model. Can it represent a complete request or
* 审查消息模型。它能否表示一个完整的请求或
* Review the stream abstraction. This is the type of object, such as
* 审查流抽象层。这是一种对象类型，例如
* Check treatment of buffers. Does the library manage the buffers
* 检查缓冲区处理方式。该库是否负责管理缓冲区
* How does the library handle corner cases such as trailers,
* 该库如何处理尾部（trailers）等边界情况，
Declarations examples from external libraries have been edited:
来自外部库的声明示例已做编辑：
cpp-netlib
cpp-netlib
is a network programming library previously intended for Boost but not having gone through formal review. As of this writing it still uses the Boost name, namespace, and directory structure although the project states that Boost acceptance is no longer a goal. The library is based on Boost.Asio and bills itself as ['"a collection of network related routines/implementations geared towards providing a robust cross-platform networking library"]. It cites ['"Common Message Type"] as a feature. As of the branch previous linked, it uses these declarations: ``` template <class Tag> struct basic_message {
cpp-netlib 是一个网络编程库，原本计划纳入 Boost，但并未通过正式审查。在撰写本文时，它仍然使用 Boost 的名称、命名空间和目录结构，尽管该项目声明不再将纳入 Boost 作为目标。该库基于 Boost.Asio，自称是“一组与网络相关的例程/实现，旨在提供一个健壮的跨平台网络库”。它将“通用消息类型”列为其特性之一。在上述链接的分支中，它使用了以下声明：
``` template struct basic_message {
82
}; ```
}; ```
This container is the base class template used to represent HTTP messages. It uses a "tag" type style specializations for a variety of trait classes, allowing for customization of the various parts of the message. For example, a user specializes `headers_container<T>` to determine what container type holds the header fields. We note some problems with the container declaration:
这个容器是用于表示 HTTP 消息的基类模板。它使用“标签（tag）”类型风格来特化各种特征类，从而允许对消息的各个部分进行自定义。例如，用户可以通过特化 `headers_container<T>` 来确定使用何种容器类型来存储头部字段。我们注意到该容器的声明存在一些问题：
* The header and body containers may only be default-constructed.
* 头部和主体容器只能通过默认构造函数进行构造。
* No stateful allocator support.
* 不支持有状态分配器。
* There is no way to defer the commitment of the type for `body_` to
* 无法推迟 `body_` 类型的提交。
* The message model includes a "source" and "destination." This is
* 消息模型包含了“源”和“目标”。这是
* The use of `string_type` (a customization point) for source,
* 对源（source）使用 `string_type`（一个自定义点），
* The library uses specializations of `string<Tag>` to change the type
* 该库使用 `string<Tag>` 的特化来改变类型
* The specialized trait classes generate a proliferation of small
* 特化的特征类会导致大量小型
* The `string<Tag> customization point constrains user defined body types
* `string<Tag>` 自定义点限制了用户自定义主体类型
English Chinese (Simplified Han script) Actions
Tasks in the Concurrency Runtime operate in a fashion similar to `std::future`, but with some improvements such as continuations which are not yet in the C++ standard. The costs of using a task based asynchronous interface instead of completion handlers is well documented: synchronization points along the call chain of composed task operations which cannot be optimized away. See: [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3747.pdf [*A Universal Model for Asynchronous Operations]] (Kohlhoff).
Concurrency Runtime 中的任务操作方式与 `std::future` 类似，但进行了一些改进，例如 C++ 标准中尚未包含的 continuation（续体）。使用基于任务的异步接口而非完成处理程序（completion handlers）的代价已有详细记录：在组合任务操作的调用链上存在无法优化的同步点。参见：[@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3747.pdf [*A Universal Model for Asynchronous Operations]] (Kohlhoff)。
} // namespace details ```
} // namespace details ```
* There is no way to defer the commitment of the type for `body_` to
* 无法推迟 `body_` 类型的提交。
The general strategy used by the author to evaluate external libraries is as follows:
作者评估外部库的总体策略如下：
* The specialized trait classes generate a proliferation of small
* 特化的特征类会导致大量小型
is a Microsoft project which ['"...aims to help C++ developers connect to and interact with services"]. It offers the most functionality of the libraries reviewed here, including support for Websocket services using its websocket++ dependency. It can use native APIs such as HTTP.SYS when building Windows based applications, and it can use Boost.Asio. The WebSocket module uses Boost.Asio exclusively.
是一个微软项目，其目标是“帮助 C++ 开发者连接服务并与之交互”。在本文所评测的库中，它的功能最为全面，包括通过其 websocket++ 依赖项来支持 WebSocket 服务。在构建基于 Windows 的应用程序时，它能够使用诸如 HTTP.SYS 等原生 API，同时也支持使用 Boost.Asio。其中，WebSocket 模块专门且排他性地使用了 Boost.Asio。
* The constructor accepting `std::unique_ptr<http::details::_http_server_context`
* 接受 `std::unique_ptr<http::details::_http_server_context` 的构造函数
* The `_reply_impl` function implies that the message implementation also
* `_reply_impl` 函数意味着消息的实现也
* The `string<Tag> customization point constrains user defined body types
* `string<Tag>` 自定义点限制了用户自定义主体类型
As cpprestsdk is developed by a large corporation, it contains quite a bit of functionality and necessarily has more interfaces. We will break down the interfaces used to model messages into more manageable pieces. This is the container used to store the HTTP header fields: ``` class http_headers { public:
由于 cpprestsdk 由大型公司（微软）开发，它包含了相当多的功能，因此也必然拥有更多的接口。我们将把用于建模消息的接口拆解为更易于管理的部分。以下是用于存储 HTTP 头字段的容器： ``` class http_headers { public:
* How does the library handle corner cases such as trailers,
* 该库如何处理尾部（trailers）等边界情况，
* No way to define an allocator for the container used when extracting
* 无法为提取数据时所使用的容器定义分配器
* There are no compile-time customization points at all. The only
* 完全没有编译期自定义点。唯一
There are a few C++ published libraries which implement some of the HTTP protocol. We analyze the message model chosen by those libraries and discuss the advantages and disadvantages relative to Beast.
目前已有一些 C++ 库实现了部分 HTTP 协议。我们将分析这些库所采用的消息模型，并讨论它们相对于 Beast 的优缺点。
* Review the message model. Can it represent a complete request or
* 审查消息模型。它能否表示一个完整的请求或
* Review the stream abstraction. This is the type of object, such as
* 审查流抽象层。这是一种对象类型，例如
* Check treatment of buffers. Does the library manage the buffers
* 检查缓冲区处理方式。该库是否负责管理缓冲区
Declarations examples from external libraries have been edited:
来自外部库的声明示例已做编辑：
cpp-netlib
cpp-netlib
is a network programming library previously intended for Boost but not having gone through formal review. As of this writing it still uses the Boost name, namespace, and directory structure although the project states that Boost acceptance is no longer a goal. The library is based on Boost.Asio and bills itself as ['"a collection of network related routines/implementations geared towards providing a robust cross-platform networking library"]. It cites ['"Common Message Type"] as a feature. As of the branch previous linked, it uses these declarations: ``` template <class Tag> struct basic_message {
cpp-netlib 是一个网络编程库，原本计划纳入 Boost，但并未通过正式审查。在撰写本文时，它仍然使用 Boost 的名称、命名空间和目录结构，尽管该项目声明不再将纳入 Boost 作为目标。该库基于 Boost.Asio，自称是“一组与网络相关的例程/实现，旨在提供一个健壮的跨平台网络库”。它将“通用消息类型”列为其特性之一。在上述链接的分支中，它使用了以下声明：
``` template struct basic_message {
* The use of `string_type` (a customization point) for source,
* 对源（source）使用 `string_type`（一个自定义点），
is a library resulting from the 2014 Google Summer of Code. It was submitted for a Boost formal review and rejected in 2015. It is based on Boost.Asio, and development on the library has continued to the present. As of the branch previously linked, it uses these message declarations: ``` template<class Headers, class Body> struct basic_message {
是一个源自 2014 年 Google Summer of Code 的库。它曾于 2015 年提交进行 Boost 正式审查，但遭到拒绝。该库基于 Boost.Asio 构建，其开发工作一直持续至今。在之前链接的分支中，它使用了如下的消息声明：``` template struct basic_message {
typedef basic_message<boost::http::headers, std::vector<std::uint8_t>> message;
typedef basic_message<boost::http::headers, std::vector<std::uint8_t>> message;
template<class Headers, class Body> struct is_message<basic_message<Headers, Body>>: public std::true_type {}; ```
template<class Headers, class Body> struct is_message<basic_message<Headers, Body>>: public std::true_type {}; ```
* `headers_`, `body_`, and `trailers_` may only be default-constructed,
* `headers_`、`body_` 和 `trailers_` 只能进行默认构造，
* There is no way to defer the commitment of the [*Body] type to after
* 无法将 [*Body] 类型的提交推迟到之后进行
* No stateful allocator support. This follows from the previous limitation
* 不支持有状态分配器。这是由前一个限制导致的
* The trailers are stored in a separate object. Aside from the combinatorial
* 尾部（trailers）存储在一个单独的对象中。除了组合爆炸的问题之外
This representation addresses a narrow range of use cases. It has limited potential for customization and performance. It is more difficult to use because it excludes the start line fields from the model.
* 这种表示法仅能解决一小部分用例。其自定义潜力和性能表现均十分有限。由于该模型将起始行（start line）字段排除在外，导致其使用难度增加。
This declaration is quite bare-bones. We note the typical problems of most field containers:
这个声明相当简陋。我们注意到大多数字段容器都存在一些典型问题：

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Translation changed
* Review the stream abstraction. This is the type of object, such as
* Review the stream abstraction. This is the type of object, such as审查流抽象层。这是一种对象类型，例如
06/20/2026
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* Review the stream abstraction. This is the type of object, such as
* Review the stream abstraction. This is the type of object, such as
06/05/2026
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type: list
Unit identifier
58620
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string ID 23
String age
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Last updated
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doc/qbk/08_design/2_http_comparison_zh_Hans.qbk, string 5