正确使用'extern'关键字 [英] Using the 'extern' keyword properly

问题描述

有些资料(书籍，在线材料)解释了 extern 的用法，如下所示:

There are sources (books, online materials) that explain the usage of extern as following:

extern int i;        // declaration - has 'extern'
int i = 1;           // definition  - specified by the absence of 'extern'

有些来源支持以下语法:

And there are sources that support the following syntax:

extern int i;        // declaration
extern int i = 1;    // definition  - specified by the equal sign
                     // Both marked with 'extern'

我的问题是-这是 C 与 C ++ 的区别，还是 pre-ANSI 与 ANSI的区别练习?

My question is - is this a C vs. C++ distinction, or is it a pre-ANSI vs. ANSI practice?

现在，更实际的问题:

使用第二种语法，我想创建一个全局对象(从每个编译单元可见).构造函数不带参数，因此不需要括号或等号.

Using the second syntax, I want to create a global object (visible from every compilation unit). The constructor takes no parameters, so neither parentheses, nor the equal sign are necessary.

extern MyClass myobject;

现在编译器如何区分声明和定义?

Now how can the compiler make the distinction between a declaration and the definition?

编辑:回到学校后，我习惯了第一种语法(Borland C).后来，我使用了一个编译器(可能是GCC的某些古老版本)，该编译器拒绝在没有外部"的情况下编译定义.那就是让我感到困惑的原因.

Back at school, I was used to the first syntax (Borland C). Later I used a compiler (probably some ancient version of GCC) that refused to compile a definition without an 'extern'. That is what made me confused.

推荐答案

专门针对您的示例，此处的C和C ++之间没有区别.两种语言都适用的基本规则是:如果声明中包含 initializer ，则它是定义.时期.不管是否有显式的 extern .如果它具有初始化程序，则它是定义.

Specifically for your examples, there's no distinction between C and C++ at work here. The basic rule that works in both languages is this: if your declaration includes an initializer, then it is a definition. Period. It does not matter, whether it has explicit extern in it or not. If it has an initializer, then it is a definition.

这意味着在命名空间范围内， extern int i = 1 和 int i = 1 都是等效的，即，这样声明中的 extern 是多余的.在C ++中，当声明的对象为 const 时，定义中的 extern 变得非冗余，因为默认情况下C ++中的 const 对象具有内部链接.例如， extern const int c = 42; 定义具有外部链接的常量 c .

That means that in namespace scope both extern int i = 1 and int i = 1 are equivalent, i.e. extern in such declaration is redundant. In C++ extern in definitions becomes non-redundant when the declared object is const, since const objects in C++ have internal linkage by default. For example, extern const int c = 42; defines constant c with external linkage.

如果声明没有初始化程序，则(并且仅在此之前)它开始取决于 extern 关键字的存在.对于 extern ，它是一个未定义的声明.如果没有 extern ，则为定义.(在C语言中，这是一个暂定的定义，但这在我们的上下文中是不重要的.)

If a declaration has no initializer, then (and only then) it begins to depend on the presence of extern keyword. With extern it is a non-defining declaration. Without extern it is a definition. (In C it would be a tentative definition, but that's beside the point in our context).

现在，对于您的实际问题.为了创建全局对象，您必须声明为

Now, for your practical question. In order to create a global object, you have to declare it as

extern MyClass myobject;

(通常将在头文件中完成)，然后在某些翻译单元中将其定义为

(which will usually be done in a header file), and then define it in some translation unit as

MyClass myobject;

由于构造函数不接受任何参数，因此这是定义对象的唯一方法.(如果需要，从C ++ 11开始，您还可以使用 MyClass myobject {}; .)

Since your constructor takes no arguments, this is the only way to define your object. (Starting from C++11 you can also use MyClass myobject{}; if you so desire.)

如果您必须向构造函数提供参数(例如 42 )，则可以同时使用两者

If you had to supply arguments to the constructor (say, 42), you would be able to use both

MyClass myobject(42);

和

extern MyClass myobject(42);

作为定义，因为存在初始化程序可确保确实将其解释为定义.

as definition, since presence of an initializer ensures that it is indeed interpreted as a definition.

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