typecast BOOL在iOS 8.1.1中总是返回false [英] typecast BOOL always returns false in iOS 8.1.1

问题描述

我保存&使用以下方法在字典中检索BOOL值。

I save & retrive the BOOL value in a dictionary using the following methods.

[userData setObject:@YES forKey:@"IS_AGENT"];

(BOOL) [userData objectForKey:@"IS_AGENT"];

在上面的代码中，从字典重试bool值我总是得到假在以下设备iPhone5s ，iPhone5c，iPhone6，iPhone6 +，它有iOS 8.1.1和相同的代码工作正常在iPod Touch有相同的iOS 8.1.1

In the above code while retrying the bool value from the dictionary I always get false in the following devices iPhone5s, iPhone5c, iPhone6, iPhone6+ which has iOS 8.1.1 and the same code works fine in iPod Touch which has the same iOS 8.1.1

google googled后我来知道我们不应该这样键入cast BOOL ，因为 BOOL 是 char BOOL 挤压一个大于 char 的值，

After googled I came to know that we should not type cast BOOL like this because "BOOL is a char, which is eight bits. If you try to squeeze a value larger than a char through BOOL, the compiler will happily truncate the upper bits, slicing them off."

现在我解决了这个问题，只需发送 boolValue 消息到特定对象。检查下面的工作代码。

Now I fixed the issue simply sending boolValue message to a particular object. Check the working code below.

[[userData objectForKey:@"IS_AGENT"] boolValue];

好吧，我的问题是为什么会发生在特定的设备（iPhone5s，iPhone5c，iPhone5 ，iPhone6 +），而不是在iPod touch中有相同的iOS 8.1.1？

Ok fine.My question is "Why is it happening in a particular devices (iPhone5s,iPhone5c,iPhone5,iPhone6+) only and not in iPod touch which is having the same iOS 8.1.1?"

推荐答案

这是我的猜测发生了什么在iOS 8.1.1之前。也许苹果改变了标记指针的工作方式，我没有机会来测试它，还没有找到任何参考，无论苹果是否改变了这个系统中的东西。

Here's my speculation what happened before iOS 8.1.1. Maybe Apple changed the way tagged pointers work here, I haven't had an opportunity to test it yet and haven't found any references whether Apple did change something in this system.

您正在向BOOL投射指针。因为我们在这里讨论小端序系统，当你转换为 BOOL char c>你得到1，因为字节不是0.在64位的iOS系统与64位二进制，运行时使用

You were casting a pointer to a BOOL. Since we're talking about little endian systems here, a pointer like 0x12345678 casted to char yields 0x78, when you cast to BOOL you get 1 since the byte is not 0. On 64-bit iOS systems with 64-bit binaries, the runtime uses tagged pointers.

对于标记的指针，第一个字节（第一个字节）这是最低有效数字，我们是小端）将总是有最后一个位设置标记为一个带标记的指针。如果将该字节转换为BOOL，您将始终获得1 / YES。

For tagged pointers, the first byte (which are the least significant digits, we're little endian) will always have the last bit set to mark it as a tagged pointer. If you cast that byte to BOOL you'll always get 1/YES.

但对于未标记的指针，由于对齐，最后四位始终为0。偶然的，它可能发生的其他位是0太（在我的测试中，我只看到的0x20的倍数，所以每8个对象将有0x00）。当你把它转换为 BOOL 时，你大部分时间都会得到YES，有时候是NO。

But for non-tagged pointers, due to aligning, the last four bits are always 0. And by chance it can happen that the other bits of that are are 0 too (in my tests I've only seen multiples of 0x20 so every 8th object would have 0x00). When you cast that to a BOOL you'll get YES most of the time and sometimes NO.

已经发现，查询值的正确方法是 [[userData objectForKey：@IS_AGENT] boolValue] 或简单地 [userData [@] IS_AGENT] boolValue] 如果您的部署目标是iOS> = 6。

As you already found out, the correct way to query the value is [[userData objectForKey:@"IS_AGENT"] boolValue] or simply [userData[@"IS_AGENT"] boolValue] if your deployment target is iOS >= 6.

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