C ++：关于使用空间std和cout问题 [英] C++: Questions about using namespace std and cout

问题描述

为什么需要使用命名空间std 来输入; ，以便能够使用 COUT 和 ENDL ？也有这些东西叫做;为 COUT 函数？

Why do I need to type in using namespace std; in order to be able to use cout and endl? Also what are these called; is cout a function?

有 COUT 用C？我听说这是用C ++实现，因为它在许多方面是更好的。

Is there cout in C? I heard it was implemented in C++ because it is better in many ways.

推荐答案

COUT 是 STD 命名空间和 ENDL 是（流操纵）函数在 STD 命名空间还定义的。

cout is a global object defined in the std namespace, and endl is a (stream manipulator) function also defined in the std namespace.

如果你不采取任何行动，以他们的名字导入到全局命名空间，你将不能够引用它们与不合格的标识符 COUT 和 ENDL 。你必须使用完全合格的名称：

If you take no action to import their names into the global namespace, you won't be able to refer to them with the unqualified identifiers cout and endl. You have to use the fully qualified names:

std::cout << "Hello, World!" << std::endl;

基本上，使用命名空间std 确实被注入存在于 STD 命名实体的所有名称进入全局命名空间：

Basically, what using namespace std does is to inject all the names of entities that exist in the std namespace into the global namespace:

using namespace std;
cout << "Hello, Wordl!" << endl;

但是，请记住，在全局命名空间这样的使用指令是一个 BAD 编程实践，这几乎肯定会导致邪恶的名称冲突即可。

However, keep in mind that have such a using directive in the global namespace is a BAD programming practice, which will almost certainly lead to evil name clashes.

如果您的真正的需要使用它（例如，如果你的一个功能是使用在 STD 命名空间中定义的多种功能以及写作的std :: 使得code难读），你倒是应该限制其范围的各个功能的局部范围：

If you really need to use it (e.g. if a function of yours is using many functions defined in the std namespace, and writing std:: makes the code harder to read), you should rather restrict its scope to the local scope of individual functions:

void my_function_using_a_lot_of_stuff_from_std()
{
    using namespace std;
    cout << "Hello, Wordl!" << endl;

    // Other instructions using entities from the std namespace...
}

好多了，只要这是实际的，是使用以下，创伤小的 using声明的，可以指定将的选择的进口只有名称：

Much better, as long as this is practical, is to use the following, less invasive using declarations, which will selectively import only the names you specify:

using std::cout;
using std::endl;

cout << "Hello, Wordl!" << endl;

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