Objective-C - 全局变量

发布于 2024-12-25 17:51:22 字数 169 浏览 2 评论 0原文

如何在 main.m 文件中声明一个变量，以便它在所有类中都可用？

如果我只是在 main 函数中声明它，编译器会说它在类方法中未声明。

我必须在这样的对象中声明它吗？

@public
type variable;

原文

How do I declare a variable in the main.m file so that it is available in all the classes?

If I simply declare it in the main function, the compiler says it's undeclared in the class method.

Must I declare it in an object like this?

@public
type variable;

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月下凄凉 2025-01-01 17:51:22

您所需要的只是使用普通的旧式 C 全局变量。

首先，在 main.m 中的 main 函数之前定义一个变量：

#import <...>

// Your global variable definition.
type variable;

int main() {
    ...

其次，您需要让其他源文件知道它。您需要在某个 .h 文件中声明它，并将该文件导入到您需要变量的所有 .m 文件中：

// .h file

// Declaration of your variable.    
extern type variable;

请注意，您不能在声明中为变量赋值块，否则它会成为该变量的定义，并且您会以链接器错误结束，抱怨同名的多个定义。

为了明确起见：每个变量可以声明多次（声明表明该变量存在于某处），但定义仅一次（定义实际上为该变量创建内存）。

但要注意，全局变量是一种不好的编码习惯，因为它们的值可能在任何文件中意外更改，因此您可能会遇到难以调试的错误。例如，您可以使用单例模式避免全局变量。

All you need is to use plain old C global variables.

First, define a variable in your main.m, before your main function:

#import <...>

// Your global variable definition.
type variable;

int main() {
    ...

Second, you need to let other source files know about it. You need to declare it in some .h file and import that file in all .m files you need your variable in:

// .h file

// Declaration of your variable.    
extern type variable;

Note that you cannot assign a value to variable in declaration block, otherwise it becomes a definition of that variable, and you end with linker error complaining on multiple definitions of the same name.

To make things clear: each variable can be declared multiple times (Declaration says that this variable exists somewhere), but defined only once (definition actually creates memory for that variable).

But beware, global variables are a bad coding practice, because their value may be unexpectedly changed in any of files, so you may encounter hard to debug errors. You can avoid global variables using Singleton pattern, for example.

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双手揣兜 2025-01-01 17:51:22

不太确定你为什么要这样做，但如果你愿意的话就可以。

main.m：

int someGlobal = 0; ///< Added outside any function, at the top say.

SomeClass.m：

extern int someGlobal; ///< Added at the top, outside the class implementation.

...

- (void)useGlobal {
    NSLog(@"someGlobal = %i", someGlobal);
    someGlobal = 5;
    NSLog(@"someGlobal = %i", someGlobal);
}

但是在开始使用这样的东西之前请仔细考虑！

Not really sure why you want to do it, but you could if you wanted.

main.m:

int someGlobal = 0; ///< Added outside any function, at the top say.

SomeClass.m:

extern int someGlobal; ///< Added at the top, outside the class implementation.

...

- (void)useGlobal {
    NSLog(@"someGlobal = %i", someGlobal);
    someGlobal = 5;
    NSLog(@"someGlobal = %i", someGlobal);
}

But please, think carefully before embarking on using something like this!

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