extern如何在c ++中工作？ [英] How does extern work in c++?

问题描述

这是来自< iostream> ：

namespace std 
{
  extern istream cin;       ///< Linked to standard input
  extern ostream cout;  
...

似乎使用 extern

It seems by using extern the data types defined in other namespaces will just be available?

推荐答案

extern 用于引用在不同编译单元中定义的变量（现在，您可以将编译单元视为.cpp文件）。您的示例中的语句 而不是 cin 和 cout 。它告诉编译器，这些对象的定义是在另一个编译单元中找到的（它们不声明为 extern ）。

extern is used to refer to a variable defined in a different compilation unit (for now, you can think of a compilation unit as a .cpp file). The statements in your example declare rather than define cin and cout. It is telling the compiler that the definition of these objects is found in another compilation unit (where they are not declared as extern).

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