August 2021
Intermediate to advanced
450 pages
9h 36m
Chinese
Content preview from C++程序设计：原理与实践（基础篇）（原书第2版）
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8.7.1 using声明和using指令
使用全限定名太繁琐了。例如，C++标准库的功能都定义在std名字空间中，因此可以按如下方式使用：
我们已经见过标准库中的string和cout无数次了，我们真不希望必须用它们“正确的”全限定名std：：string和std：：cout才能访问它们。如果有这么一种方法，能实现“当我说string，我的意思是std：：string”、“当我说cout，我的意思是std：：cout”等等，那就好了，就像下面这样：
这种语法结构称为using声明，它与我们常用的人名简称类似：你可以简单地用“Greg”来代表Greg Hansen，只要屋子里没有其他叫Greg的人就没问题。
有时，我们希望可以有一种更强的“简称”用来引用名字空间中的名字：“如果你在本作用域中没有发现某个名字的声明，那么就在std中寻找它。”使用using指令可以达到这个目的：
于是我们得到了下面这种常用的程序风格：
其中cin就表示std：：cin，string表示std：：string，依此类推。只要你使用std_lib_facilities.h，你就不需要担心标准库头文件和std名字空间了。 ...
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