什么是目的：“using namespace”？ [英] What's the purpose of: "using namespace"?

问题描述

有令人信服的论据反对 使用namespace std ，为什么它是否被引入到语言中？不要使用命名空间破坏命名空间的目的？为什么我想使用命名空间写？是否有任何问题，我不知道，由使用命名空间优雅地解决，也许在使用std :: swap idiom或类似的东西？

There are convincing arguments against using namespace std, so why was it introduced into the language at all? Doesn't using namespace defeat the purpose of namespaces? Why would I ever want to write using namespace? Is there any problem I am not aware of that is solved elegantly by using namespace, maybe in the lines of the using std::swap idiom or something like that?

推荐答案

一方面，这是在命名空间中使用运算符重载的方式（例如使用命名空间std :: rel_ops; 或使用命名空间boost :: assign; ）

For one thing, this is the way to use operator overloads in a namespace (e.g using namespace std::rel_ops; or using namespace boost::assign;)

简洁也是一个强有力的论据。你真的很喜欢键入和阅读 std :: placeholders :: _ 1 而不是 _1 吗？此外，当您以函数式编写代码时，您将使用 std 和 boost 命名空间中的无数对象。

Brevity is also a strong argument. Would you really enjoy typing and reading std::placeholders::_1 instead of _1? Also, when you write code in functional style, you'll be using a myriad of objects in std and boost namespace.

另一个重要用途（虽然通常不会导入整个命名空间）是启用参数相关的查找：

Another important usage (although normally one doesn't import whole namespaces) is to enable argument-dependent look-up:

template <class T>
void smart_swap(T& a, T& b)
{
    using std::swap;
    swap(a, b);
}

如果在与T相同的命名空间中某些类型的T重载交换，这将使用那个重载。如果你明确调用 std :: swap ，那么不会考虑该重载。对于其他类型，这可以回到 std :: swap 。

If swap is overloaded for some type of T in the same namespace as T, this will use that overload. If you explicitly called std::swap instead, that overload would not be considered. For other types this falls back to std::swap.

命名空间的用途，因为在歧义的情况下，您可以随时完全限定名称。

BTW, a using declaration/directive does not defeat the purpose of namespaces, since you can always fully qualify the name in case of ambiguity.

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