在目标C中，单例设计模式的通常实现包含“静态ID theInstance = nil".在某种程度上，为什么不在外面? [英] In Objective C, Usual implementation of singleton design pattern contains "static id theInstance = nil" in a method, why not outside?

问题描述

当我在目标C中进行Singleton设计模式时，我发现很多人使用下面的代码来创建它.

When i was going through Singleton design pattern in Objective C, I found lot of people using the below code to create it.

@interface Base : NSObject {} 

+(id)instance;
@end

@implementation Base

+(id) instance
{

static id theInstance = nil;

    if (theInstance == nil)
    {
        theInstance = [[self alloc] init];
    }
    return theInstance;
}

@end

在这里，我没有得到为什么我们必须在方法中将静态变量分配为nil的原因，而是可以在方法外部声明该静态变量并将其分配为nil. 因为每次调用此+ instance()方法，Instance变量都将分配为nil. 它会不会失去它所指向的先前对象吗?

Here i did not get the why do we have to assign the static variable to nil in a method instead it can be declared outside the method and assigned to nil. Because everytime this +instance() method is called, theInstance variable will be assigned to nil. Will it not lose its previous object to which it was pointing to?

我尝试调试它，令人惊讶的是，调用+ instance()方法时，它不会指向nil. 谁能解释我这里发生了什么事?

I have tried debugging it, surprisingly , it will not point to nil when +instance() method is called. Can anyone explain me whats happening here?

推荐答案

static变量仅初始化一次，而不管它们是全局范围还是局部范围.在这种情况下，甚至不需要nil-static存储类变量默认情况下为零初始化.声明:

static variables only get initialized once, regardless of if they're at global or local scope. In this case, you don't even need the nil - static storage class variables are zero-initialized by default. This declaration:

  static id theInstance;

足以与您所拥有的保持一致.

is enough to be the same as what you have there.

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