块内的静态变量 [英] Static variable inside block

问题描述

当我在这样的块内声明静态变量时会发生什么?

What happens when I declare a static variable inside a block like this?

dispatch_async(dispatch_get_main_queue(), ^{
 static NSInteger myNumber;
 // do stuff with myNumber      
});

1. 第二次触发该块会发生什么?
2. 如果该块在运行后将取消分配，myNumber怎么还在那里?
3. 这样可以吗?我的意思是，这种做法会引起任何问题，例如由于无法释放而导致块泄漏吗?

推荐答案

块规范没有明确提及如何处理块内的static变量，只是块主体是与函数主体相同的复合语句.因此，语义与在函数中声明的static变量的语义相同，即它们是全局生存期的变量，只能在声明它们的范围内通过名称直接访问.

The block specification does not explicitly mention how static variables within blocks are handled, just that the block body is a compound statement which is just the same as the body of a function. Therefore the semantics are the same as for static variables declared in a function, that is they are variables of global lifetime which are only directly accessible by name within the scope they are declared in.

每次评估 block文字(^{...})时，都会构造一个 value 块.此值包含一个标准的C函数指针，该指针指向块主体的已编译代码，就像其他任何复合语句一样，它们在编译时也会生成一次.

A block value is constructed each time a block literal (^{...}) is evaluated. This value contains a standard C function pointer to the compiled code of the block body, which like any other compound statement is generated once at compile time.

您的问题的答案就是这样:

The answers to your questions just follow from this:

1. 第二次触发该阻止会发生什么?

第二次执行带有局部static变量的函数时，发生同样的事情，函数主体看到了先前存储在变量中的值.

Same thing that happens the second time a function with a local static variable is executed, the function body sees the value previously stored in the variable.

1. 如果该块在运行后将取消分配，myNumber怎么还在那里?

因为它是块值，其中包括任何关联的捕获变量，所以将其释放；包含任何static变量的已编译代码始终存在.

Because it is the block value, which includes any associated captured variables, which is deallocated; the compiled code, which includes any static variables, always exists.

1. 这样可以吗?我的意思是，这种做法会引起任何问题，例如由于无法释放而导致块泄漏吗?

执行此操作与在函数中执行此操作相同.如果static是Objective-C对象类型，则存储在其中的引用可能会泄漏"-就像使用标准全局变量一样.除非您将对块本身的引用(直接或间接通过引用链)存储在static变量中，否则不会阻止该块值的重新分配.

Doing this is the same as doing it within a function. If the static is of Objective-C object type then references stored in it may "leak" - just as with standard global variables. The block value will not be prevented from deallocation unless you store a reference to the block itself (directly or indirectly via a chain of references) in the static variable.

HTH

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