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#. type: section title
#: 10
msgid "HTTP Comparison to Other Libraries"
msgstr "HTTP 与其他库的比较"

#. type: paragraph
#: 12
msgid ""
"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."
msgstr ""
"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。"

#. type: paragraph
#: 16
msgid ""
"The general strategy used by the author to evaluate external libraries is as "
"follows:"
msgstr "作者评估外部库的总体策略如下："

#. type: list
#: 19
msgid "* Review the message model. Can it represent a complete request or"
msgstr "* 审查消息模型。它能否表示一个完整的请求或"

#. type: list
#: 23
msgid "* Review the stream abstraction. This is the type of object, such as"
msgstr "* 审查流抽象层。这是一种对象类型，例如"

#. type: list
#: 28
msgid "* Check treatment of buffers. Does the library manage the buffers"
msgstr "* 检查缓冲区处理方式。该库是否负责管理缓冲区"

#. type: list
#: 31
msgid "* How does the library handle corner cases such as trailers,"
msgstr "* 该库如何处理尾部（trailers）等边界情况，"

#. type: paragraph
#: 35
msgid "Declarations examples from external libraries have been edited:"
msgstr "来自外部库的声明示例已做编辑："

#. type: heading
#: 41
msgid "cpp-netlib"
msgstr "cpp-netlib"

#. type: paragraph
#: 44
msgid ""
"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 {"
msgstr ""
"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：\n"
"``` template  struct basic_message {"

#. type: paragraph
#: 82
msgctxt "82"
msgid "}; ```"
msgstr "}; ```"

#. type: paragraph
#: 85
msgid ""
"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:"
msgstr ""
"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："

#. type: list
#: 91
msgid "* The header and body containers may only be default-constructed."
msgstr "* 头部和主体容器只能通过默认构造函数进行构造。"

#. type: list
#: 93
msgid "* No stateful allocator support."
msgstr "* 不支持有状态分配器。"

#. type: list
#: 95
msgid "* There is no way to defer the commitment of the type for `body_` to"
msgstr "* 无法推迟 `body_` 类型的提交。"

#. type: list
#: 98
msgid "* The message model includes a \"source\" and \"destination.\" This is"
msgstr "* 消息模型包含了“源”和“目标”。这是"

#. type: list
#: 102
msgid "* The use of `string_type` (a customization point) for source,"
msgstr "* 对源（source）使用 `string_type`（一个自定义点），"

#. type: list
#: 112
msgid "* The library uses specializations of `string<Tag>` to change the type"
msgstr "* 该库使用 `string<Tag>` 的特化来改变类型"

#. type: list
#: 121
msgid "* The specialized trait classes generate a proliferation of small"
msgstr "* 特化的特征类会导致大量小型"

#. type: list
#: 127
msgid ""
"* The `string<Tag> customization point constrains user defined body types"
msgstr "* `string<Tag>` 自定义点限制了用户自定义主体类型"

#. type: paragraph
#: 132
msgid ""
"The design of the message container in this library is cumbersome with its "
"system of customization using trait specializations. The use of these "
"customizations is extremely limited due to the way they are used in the "
"container declaration, making the design overly complex without "
"corresponding benefit."
msgstr ""
"该库消息容器的设计非常繁琐，其自定义系统依赖于特征（trait）特化。由于这些自定"
"义机制在容器声明中的使用方式极为受限，导致该设计过于复杂，却未能带来相应的收"
"益。"

#. type: heading
#: 140
msgid "Boost.HTTP"
msgstr "Boost.HTTP"

#. type: paragraph
#: 143
msgid ""
"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 {"
msgstr ""
"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 {"

#. type: paragraph
#: 166
msgctxt "166"
msgid "private:"
msgstr "private:"

#. type: paragraph
#: 170
msgctxt "170"
msgid "};"
msgstr "};"

#. type: paragraph
#: 172
msgid ""
"typedef basic_message<boost::http::headers, std::vector<std::uint8_t>> "
"message;"
msgstr ""
"typedef basic_message<boost::http::headers, std::vector<std::uint8_t>> "
"message;"

#. type: paragraph
#: 174
msgid ""
"template<class Headers, class Body> struct is_message<basic_message<Headers, "
"Body>>: public std::true_type {}; ```"
msgstr ""
"template<class Headers, class Body> struct is_message<basic_message<Headers, "
"Body>>: public std::true_type {}; ```"

#. type: list
#: 178
msgid ""
"* This container cannot model a complete message. The ['start-line] items"
msgstr "* 该容器无法对完整的消息进行建模。['start-line] 项"

#. type: list
#: 193
msgid "* `headers_`, `body_`, and `trailers_` may only be default-constructed,"
msgstr "* `headers_`、`body_` 和 `trailers_` 只能进行默认构造，"

#. type: list
#: 196
msgid "* There is no way to defer the commitment of the [*Body] type to after"
msgstr "* 无法将 [*Body] 类型的提交推迟到之后进行"

#. type: list
#: 200
msgid ""
"* No stateful allocator support. This follows from the previous limitation"
msgstr "* 不支持有状态分配器。这是由前一个限制导致的"

#. type: list
#: 204
msgid ""
"* The trailers are stored in a separate object. Aside from the combinatorial"
msgstr "* 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"

#. type: list
#: 212
msgid "* The declarations imply that `std::vector` is a model of [*Body]."
msgstr "* 这些声明暗示 `std::vector` 是 [*Body] 的一个模型。"

#. type: list
#: 223
msgid "* The [*Body] customization point constrains user defined types to"
msgstr "* [*Body] 自定义点将用户自定义类型限制为"

#. type: paragraph
#: 228
msgid ""
"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."
msgstr ""
"* 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。"

#. type: heading
#: 234
msgid "C++ REST SDK (cpprestsdk)"
msgstr "C++ REST SDK (cpprestsdk)"

#. type: paragraph
#: 237
msgid ""
"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."
msgstr ""
"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。"

#. type: paragraph
#: 244
msgid ""
"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:"
msgstr ""
"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:"

#. type: paragraph
#: 254
msgctxt "254"
msgid "private:"
msgstr "private:"

#. type: paragraph
#: 256
msgctxt "256"
msgid "}; ```"
msgstr "}; ```"

#. type: paragraph
#: 259
msgid ""
"This declaration is quite bare-bones. We note the typical problems of most "
"field containers:"
msgstr "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："

#. type: list
#: 262
msgid "* The container may only be default-constructed."
msgstr "* 该容器只能进行默认构造。"

#. type: list
#: 264
msgid "* No support for allocators, stateful or otherwise."
msgstr "* 不支持分配器，无论是有状态分配器还是其他类型的分配器。"

#. type: list
#: 266
msgid "* There are no customization points at all."
msgstr "* 完全没有任何自定义点。"

#. type: paragraph
#: 268
msgid ""
"Now we analyze the structure of the larger message container. The library "
"uses a handle/body idiom. There are two public message container interfaces, "
"one for requests (`http_request`) and one for responses (`http_response`). "
"Each interface maintains a private shared pointer to an implementation "
"class. Public member function calls are routed to the internal "
"implementation. This is the first implementation class, which forms the base "
"class for both the request and response implementations: ``` namespace "
"details {"
msgstr ""
"现在，我们来分析更大的消息容器的结构。该库使用了句柄/主体惯用法（handle/body "
"idiom）。它提供了两个公开的消息容器接口，一个用于请求（`http_request`），另一"
"个用于响应（`http_response`）。每个接口都维护着一个指向实现类的私有共享指针。"
"公开的成员函数调用会被路由到内部的实现。以下是第一个实现类，它构成了请求和响"
"应实现类的基类： ``` namespace details {"

#. type: paragraph
#: 279
msgid "class http_msg_base { public:"
msgstr "class http_msg_base { public:"

#. type: paragraph
#: 294
msgid "protected:"
msgstr "protected:"

#. type: paragraph
#: 306
msgctxt "306"
msgid "}; ```"
msgstr "}; ```"

#. type: paragraph
#: 309
msgid ""
"To understand these declarations we need to first understand that cpprestsdk "
"uses the asynchronous model defined by Microsoft's [@https://"
"msdn.microsoft.com/en-us/library/dd504870.aspx [*Concurrency Runtime]]. "
"Identifiers from the [@https://msdn.microsoft.com/en-us/library/"
"jj987780.aspx [*`pplx` namespace]] define common asynchronous patterns such "
"as tasks and events. The `concurrency::streams::istream` parameter and "
"`m_data_available` data member indicates a lack of separation of concerns. "
"The representation of HTTP messages should not be conflated with the "
"asynchronous model used to serialize or parse those messages in the message "
"declarations."
msgstr ""
"要理解这些声明，需要先了解 cpprestsdk 使用了微软的 [@https://"
"msdn.microsoft.com/en-us/library/dd504870.aspx [*Concurrency Runtime]] 所定义"
"的异步模型。来自 [@https://msdn.microsoft.com/en-us/library/jj987780.aspx "
"[*`pplx` namespace]] 的标识符定义了任务和事件等通用异步模式"
"。`concurrency::streams::istream` 参数和 `m_data_available` 数据成员表明存在"
"关注点分离缺失的问题。在消息声明中，不应将 HTTP 消息的表示形式与用于序列化或"
"解析这些消息的异步模型混为一谈。"

#. type: paragraph
#: 319
msgid ""
"The next declaration forms the complete implementation class referenced by "
"the handle in the public interface (which follows after): ``` /// Internal "
"representation of an HTTP request message. class _http_request final : "
"public http::details::http_msg_base, public "
"std::enable_shared_from_this<_http_request> { public:"
msgstr ""
"以下声明构成了公开接口中的句柄所引用的完整实现类（该类位于后续）：``` /// "
"HTTP 请求消息的内部表示。 class _http_request final : public "
"http::details::http_msg_base, public "
"std::enable_shared_from_this<_http_request> { public:"

#. type: paragraph
#: 336
msgctxt "336"
msgid "private:"
msgstr "private:"

#. type: paragraph
#: 344
msgctxt "344"
msgid "};"
msgstr "};"

#. type: paragraph
#: 346
msgid "} // namespace details ```"
msgstr "} // namespace details ```"

#. type: paragraph
#: 349
msgid ""
"As before, we note that the implementation class for HTTP requests concerns "
"itself more with the mechanics of sending the message asynchronously than it "
"does with actually modeling the HTTP message as described in __rfc7230__:"
msgstr ""
"与之前一样，可以看出，HTTP 请求的实现类更关注于异步发送消息的机制，而不是按"
"照 __rfc7230__ 中的描述对 HTTP 消息进行实际建模："

#. type: list
#: 353
msgid ""
"* The constructor accepting "
"`std::unique_ptr<http::details::_http_server_context`"
msgstr "* 接受 `std::unique_ptr<http::details::_http_server_context` 的构造函数"

#. type: list
#: 357
msgid ""
"* The \"cancellation token\" is stored inside the message. This breaks the"
msgstr "* “取消令牌”被存储在消息内部。这破坏了"

#. type: list
#: 360
msgid ""
"* The `_reply_impl` function implies that the message implementation also"
msgstr "* `_reply_impl` 函数意味着消息的实现也"

#. type: paragraph
#: 364
msgid ""
"Finally, here is the public class which represents an HTTP request: ``` "
"class http_request { public:"
msgstr "最后，下面是代表 HTTP 请求的公共类：``` class http_request { public:"

#. type: paragraph
#: 407
msgctxt "407"
msgid "private:"
msgstr "private:"

#. type: paragraph
#: 412
msgctxt "412"
msgid "}; ```"
msgstr "}; ```"

#. type: paragraph
#: 415
msgid ""
"It is clear from this declaration that the goal of the message model in this "
"library is driven by its use-case (interacting with REST servers) and not to "
"model HTTP messages generally. We note problems similar to the other "
"declarations:"
msgstr ""
"从这个声明中可以清楚地看出，该库中消息模型的目标是由其用例（与 REST 服务器交"
"互）驱动的，而不是为了对 HTTP 消息进行通用建模。我们注意到存在与其他声明类似"
"的问题："

#. type: list
#: 420
msgid "* There are no compile-time customization points at all. The only"
msgstr "* 完全没有编译期自定义点。唯一"

#. type: list
#: 426
msgid "* The extraction of the body is conflated with the asynchronous model."
msgstr "* 消息体的提取与异步模型混为一谈。"

#. type: list
#: 428
msgid "* No way to define an allocator for the container used when extracting"
msgstr "* 无法为提取数据时所使用的容器定义分配器"

#. type: list
#: 431
msgid "* A body can only be extracted once, limiting the use of this container"
msgstr "* 消息体只能提取一次，这限制了该容器的使用"

#. type: list
#: 434
msgid ""
"* Setting the body requires either a vector or a "
"`concurrency::streams::istream`."
msgstr "* 设置消息体需要使用向量或 `concurrency::streams::istream`。"

#. type: list
#: 437
msgid "* The HTTP request container conflates HTTP response behavior (see the"
msgstr "* HTTP 请求容器混淆了 HTTP 响应行为（见"

#. type: paragraph
#: 442
msgid ""
"The general theme of the HTTP message model in cpprestsdk is \"no user "
"definable customizations\". There is no allocator support, and no separation "
"of concerns. It is designed to perform a specific set of behaviors. In other "
"words, it does not follow the open/closed principle."
msgstr ""
"cpprestsdk 中 HTTP 消息模型的总体主题是“不支持用户可定义的自定义功能”。它没有"
"分配器支持，也没有关注点分离。它的设计目的是执行特定的行为集。换句话说，它没"
"有遵循开闭原则。"

#. type: paragraph
#: 447
msgid ""
"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)."
msgstr ""
"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)。"