为什么命名空间组成这么少使用？ [英] Why is namespace composition so rarely used?

问题描述

在他的书 C ++编程语言（第三版）中，Stroustrup教会在自己的命名空间中定义各个组件，并将它们导入到一般的命名空间中。

例如：

 命名空间array_api {
 struct array {}; 
 void print（const array&）{} 
} 
 
命名空间list_api {
 struct list {}; 
 void print（const list&）{} 
} 
 
命名空间api {
使用array_api :: array; 
 using list_api :: list; 
} 
  

我看起来很有趣，但我从来没有见过这种方法。 / p>

为什么这种技术几乎从不使用？

解决方案

不知道什么好处（如Raymond Chen所说，每个功能从-100点开始）。然而，我有一个对立点提供：Luabind，它使用的东西，看起来像这样。请参见 luabind / object.hpp ，其中实质上说：

  namespace luabind {
 namespace adl {
 class object {
}; 
} 
使用adl :: object; 
} 
  

从这个名字我们可以推断出动机： 。给定用户知道的 luabind :: object （实际上是 luabind :: adl :: object 相关函数将由编译器从 luabind :: adl 命名空间中自动发现。然而，这些函数，用户可能不需要以非常明确的方式知道，不污染主要的 luabind 命名空间。所以这很好，我猜。

但你知道什么不好吗？转发声明这些类之一。这失败：

  namespace luabind {class object; } 
  

您需要这样做：

  namespace luabind {namespace adl {class object; }} 
  

因此，抽象会迅速泄露，因为用户需要知道这个魔法 adl 命名空间。

In his book The C++ Programming Language (third edition) Stroustrup teaches to define individual components in their own namespace and import them in a general namespace.

For example:

namespace array_api {    
    struct array {};    
    void print(const array&) { }
}

namespace list_api {
    struct list {};        
    void print(const list&) { }
}

namespace api {
    using array_api::array;
    using list_api::list;
}

I looks interesting, but I have never seen this approach used in practice.

Why is this technique is almost never used?

解决方案

Mostly I wonder what the benefits would be (as Raymond Chen says, every feature starts with -100 points). However, I have a counterpoint to offer: Luabind, which does use something that looks like this. See luabind/object.hpp, which essentially says:

namespace luabind {
  namespace adl {
    class object {
    };
  }
  using adl::object;
}

From the name alone we can infer the motivation: to support argument-dependent lookup. Given what the user knows as a luabind::object, which is actually a luabind::adl::object, related functions will be discovered automatically by the compiler from the luabind::adl namespace. Yet those functions, which the user may not need to know about in a very explicit way, do not "pollute" the main luabind namespace. So that's nice, I guess.

But you know what's not nice? Forward declaring one of these classes. This fails:

namespace luabind { class object; }

You need to do this instead:

namespace luabind { namespace adl { class object; } }

Thus the abstraction quickly leaks, as the user does need to know about this magic adl namespace after all.

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