为什么 dispatch_queue_create 在 Swift 中会给出 EXC_BAD_ACCESS 错误?

问题描述

我正在将一些代码从 C++ 移植到使用 Grand Central Dispatch 的 Swift，我发现 dispatch_queue_create 的一个奇怪错误似乎根本不起作用.

I am porting some code from C++ to Swift that used Grand Central Dispatch, and I am finding a curious error with dispatch_queue_create seemingly not working at all.

例如，在我的 C++ 基类头文件中，我会声明

For instance, in my C++ base class header, I would declare

dispatch_queue_t m_WorkQ;

然后在初始化器中放入

m_ResultQ = dispatch_queue_create("com.myapp.mHitsUpdateQueue", 0);

...一切都很美好.

我已经在 Swift 中尝试过，在我的班级中，在班级级别声明了这一点:

I've tried this in Swift, in my class, declaring this at class level:

var resultQueue: dispatch_queue_t

...在初始化程序中，我(除其他外)有一行

... and in the initalizer, I have (among others) the line

resultQueue = dispatch_queue_create("com.myapp.mHitsUpdateQueue", 0)

...它编译并启动正常，但在上面的行上给我一个 EXC_BAD_ACCESS (code=1, address = 0x37) 的即时运行时错误

... and it compiles and starts up fine, but gives me an immediate runtime error of EXC_BAD_ACCESS (code=1, address = 0x37) on the above line

为了确定它是否是我所做的其他任何事情，我创建了一个命令行工具应用程序，仅包含以下代码:

To determine if it's anything else I've done, I created a command line tool application consisting only of the following code:

import Foundation

var thisQueue = dispatch_queue_create("com.myApp.mHitsUpdateQueue", 0)

println(thisQueue.description)

...果然，我在thisQueue"分配行上得到了上述错误.所以我很确定我缺少关于 Swift 和 GCD 队列创建的一些非常明显的东西.

... and sure enough, I get the above error right on the "thisQueue" assignment line. So I'm pretty sure there's something really obvious about Swift and GCD queue creation that I'm missing.

谁能帮帮我?

推荐答案

dispatch_queue_create()的第二个参数有类型dispatch_queue_attr_t，声明为

The second argument of dispatch_queue_create() has the type dispatch_queue_attr_t, which is declared as

typealias dispatch_queue_attr_t = NSObject

您必须为串行队列传递 DISPATCH_QUEUE_SERIAL 或 nil(或 DISPATCH_QUEUE_CONCURRENT 用于并发队列):

You have to pass DISPATCH_QUEUE_SERIAL or nil for a serial queue (or DISPATCH_QUEUE_CONCURRENT for a concurrent queue):

var thisQueue = dispatch_queue_create("com.myApp.mHitsUpdateQueue", DISPATCH_QUEUE_SERIAL)

在 C(++) 中，可以传递 0 而不是 NULL 指针.

In C(++), 0 can be passed instead of a NULL pointer.

然而，Swift 编译器将整数 0 包装到 NSNumber 对象中以便它可以传递给期望 NSObject 的函数范围.这会导致运行时异常，因为 NSNumber 是不是有效的属性.所以传递 0 或 nil 是在 Swift 中明显不同.

The Swift compiler, however, wraps the integer 0 into an NSNumber object so that it can be passed to the function expecting an NSObject parameter. That causes the runtime exception because NSNumber is not a valid attribute. So passing 0 or nil is significantly different in Swift.

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