C++ 使用命名空间中的符号 [英] C++ Using a symbol from a namespace

问题描述

我正在学习 websocketpp 仅标头库的工作原理，我对到处散布的 lib::error_code 类型感到非常困惑.Xcode 只会向我显示类型的声明在 <system_error> 标头中，这有点令人困惑，因为我看到 lib 是一个 websocketpp 命名空间.

I am learning how the websocketpp header-only library works, and I was quite perplexed by the lib::error_code type that is sprinkled everywhere. Xcode would only reveal to me that the declaration of the type is in the <system_error> header, which was somewhat confusing, because I saw that lib is a websocketpp namespace.

然后我最终在 websocketpp/common/system_error.hpp 中找到了这个:

Then eventually I find this in websocketpp/common/system_error.hpp:

namespace websocketpp {
namespace lib {

#ifdef _WEBSOCKETPP_CPP11_SYSTEM_ERROR_
    using std::error_code;

这是一个命名空间内的using，这不是我理解的.

This is a using inside of a namespace, which is not something that I understood.

这是做什么的?这是否只是将 websocketpp::lib::error_code 别名为 std::error_code?

What does this do? Does this simply alias websocketpp::lib::error_code to be std::error_code?

如果是这样，为什么不将其声明为 typedef std::error_code error_code?这对我来说更有意义.

If so, why is this not delared as typedef std::error_code error_code? That would make more sense to me.

推荐答案

(此处为 WebSocket++ 库作者)using 允许将模板类别名为 C++98 中的命名空间.typedef(c++11 之前的)需要一个完全指定的类型.

(WebSocket++ library author here) using allows aliasing a template class into a namespace in C++98. typedef (pre-c++11) requires a fully specified type.

对于 lib::error_code 这无关紧要，因为它不是模板类，而是命名空间 websocketpp::lib 用于别名的一般模式boost:: 和 std:: 之间要求它也适用于模板类(例如 lib::shared_ptr).为了一致性，websocketpp::lib 中的所有别名都使用 using 语法，而不是在非模板类的 typedef 和 之间切换使用作为模板.

In the case of lib::error_code this is irrelevant because it is not a template class, but the general pattern that the namespace websocketpp::lib uses to alias between boost:: and std:: requires that it work for template classes as well (e.g. lib::shared_ptr). For consistencies sake, all aliases in websocketpp::lib use the using syntax rather than switching between typedef for non-template classes and using for templates.

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