只能在变量中“声明"变量.全局，但不能修改/(单独初始化)

问题描述

免责声明:

• 这可能是一个非常琐碎的问题(尽管我找不到答案)，并且
• 一个纯粹的理论问题(我从来不需要这样做，也从来没有见过使用这种结构的代码，但是我很好奇这种方式是/为什么发生的.)
• C/C++双重标记，因为我在C和C ++上都测试了一下代码，它只是4行代码(而且唯一的区别是gcc/clang在g++/clang++时发出警告给出一个错误.)

• This might be a very trivial question (although I cannot find an answer for it), and
• a purely theoretical question (I never needed to do this, nor ever saw code that uses such constructs, but I am just curious how/why this happens the way it does.)
• The C/C++ double tag, because I tested the bit of code on both C and C++ and it is just 4 lines of code (and the only difference is that gcc/clang give a warning while g++/clang++ give an error.)

背景:在回答另一个问题时，我开始了考虑一下OP为什么不能修改一个public static变量.我考虑了一下，然后进一步减少了问题，可以看到相同的效果，但不需要任何类或静态成员变量.

Background:In replying to another question, I started to think about why the OP can not modify a public static variable. I thought about it a bit and then reduced the problem a bit further, where I can see the same effect, but without needing any class or static member variables.

问题:然后以下代码可以重现观察结果.

Question: Then the following code can reproduce the observation.

int global_n; // I know it can be initialized right away here also: int global_n = 1;
global_n = 2; // This does not compile in C++. In C it gives a warning about missing type-specifier

int main() {
    global_n = 2; // This does compile in both C/C++ of course
}

1. 这使我想到了一个问题:全局变量(因此是static变量/成员变量)只能在声明它们时直接在那里初始化.但是任何后续修改只能在函数内部进行.正确吗?

1. Which brings me to my question: Global variables (and hence static variables/member-variables) can only be initialized, directly there when they are declared. But any subsequent modifications can only occur inside a function. Correct?

是否有任何特定原因?

推荐答案

在函数之外，您不能有语句(即可执行的代码行)，只有声明和定义.

Outside of a function, you cannot have statements (i.e. executable lines of code), only declarations and definitions.

对于在全局范围内的global_n = 2;，C90具有一个遗留功能，即如果声明的变量没有类型，则其默认类型为int(C99删除了该功能并需要一个类型).这就是这种情况，这也是为什么您收到有关类型丢失的警告的原因.

In the case of global_n = 2; at global scope, C90 has a legacy feature that if a variable is declared without a type then the has a default type of int (C99 removed the feature and requires a type). That's what's happening in this case, and that's also why you get a warning about the type missing.

C ++没有该规则，因此它显示为函数之外的语句，这是错误的.

C++ doesn't have that rule, so this appears as a statement outside of a function which is an error.

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