将Objective-C对象作为void *指针传递给函数 [英] Pass an Objective-C object to a function as a void * pointer

问题描述

我有一个函数：

  myFunction (MyProc callback, void * ref)

此函数在Objective-C类中调用。函数传递一个指向回调的指针（类中的一个函数）和一个引用。引用是必要的，因为回调被静态调用，因此没有上下文。 ref可以用于为回调提供上下文。

This function is called from within an Objective-C class. The function is passed a pointer to the callback (a function in the class) and a reference. The reference is necessary because the callback is called statically and therefore doesn't have a context. The ref can be used to provide a context to the callback.

我想要能够传递Objective-C类作为参考。所以问题是：

I want to be able to pass the Objective-C class as the reference. So the question is:

如何将NSObject转换为void *，以及如何将void *作为NSObject。

How do I cast an NSObject to a void * and how do I cast a void * as an NSObject.

提前感谢。

推荐答案

执行以下操作：

void func(void *q)
{
    NSObject* o = CFBridgingRelease(q);
    NSLog(@"%@", o);
}

int main(int argc, const char * argv[])
{
    @autoreleasepool {
        NSObject* o = [NSObject new];
        func((void*)CFBridgingRetain(o));
    }
    return 0;
}

注意 CFBridgingRetain（）和 CFBridgingRelease（）是关于编译器属性的宏。随意使用。我喜欢的API变体，因为它是更常见的使用在我们的代码库，它是更明确/不太混乱。

Note that CFBridgingRetain() and CFBridgingRelease() are macros around compiler attributes. Feel free to use either. I like the API variant as it is in more common use in our codebases and it is more explicitly / less confusing.

CFBridgingRetain code>有效地硬保留必须由 CFBridgingRelease（）平衡的对象。它也碰巧返回 CFTypeRef ，其 与转换兼容 void * 。 CFBridgingRelease（）有效地撤销硬保留，因此 q c $ c> o 有效。

CFBridgingRetain() effectively hard-retains the object that must be balanced by a CFBridgingRelease(). It also happens to return a CFTypeRef which is compatible with a cast to void*. CFBridgingRelease() effectively undoes that hard-retain and, thus, q will only remain valid within the scope that o is valid.

对基本回调有效，但是你可能不会用 void * context; 类型的东西，必须坚持一段时间。为此：

Valid for basic callbacks, but you'd probably not what that with a void *context; type thing that has to stick around for a while. For that:

void callback(void *context)
{
    // grab an ARC aware reference without impacting hard-retain
    NSObject* o = (__bridge NSObject *)(context);
    NSLog(@"%@", o);
}

void freeContext(void *context)
{
    // release the hard-retain
    CFBridgingRelease(context);
}

请注意，Xcode是非常好的建议你应该做什么，如果你离开out调用cast / API调用。它甚至解释了每个替代解决方案的含义（我依赖这一点，直到我可以保持在我的头直）。

Note that Xcode is quite good about suggesting exactly what you should do if you leave out the cast / API call. It even explains the meanings of each of the alternative solutions (I relied on this heavily until I could keep 'em straight in my head).

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