如果是一个整数LT; - ＆GT;指针转换实际上是正确的？ [英] When is an integer<->pointer cast actually correct?

问题描述

常见的民间传说说：

• 该型系统中存在的一个原因。整数和指针是不同的类型，它们之间的铸造是在大多数情况下的弊端，可能表明一个设计错误，应该避免。

• The type system exists for a reason. Integers and pointers are distinct types, casting between them is a malpractice in the majority of cases, may indicate a design error and should be avoided.

即使进行这样的演员阵容，没有假设应进行有关整数和指针（铸件的尺寸无效* 到 INT 是使code失败的x64）的最简单的方法，而不是 INT 应该使用使用intptr_t 或 uintptr_t形式从 stdint.h 。

Even when such a cast is performed, no assumptions shall be made about the size of integers and pointers (casting void* to int is the simplest way to make the code fail on x64), and instead of int one should use intptr_t or uintptr_t from stdint.h.

知道了，时，它实际上有用执行这样的类型转换？

Knowing that, when is it actually useful to perform such casts?

的（注：有一点短code便携性的价格并不能算作真正有用）的

一个情况下，我知道了：

One case I know:

• 部分的无锁算法多利用一个2 + - 字节指针神韵有一些冗余的事实。然后他们使用的指针作为布尔标志的最低位，例如。用具有一个适当的指令集的处理器，这可以消除对一个锁定机构的需要（如果指针和布尔标记是分开这将是必要的）。结果，
  的（注意：这种做法甚至有可能在通过java.util.concurrent.atomic.AtomicMarkableReference的Java安全地完成）的

• Some lock-free multiprocessor algorithms exploit the fact that a 2+-byte-alligned pointer has some redundancy. They then use the lowest bits of the pointer as boolean flags, for instance. With a processor having an appropriate instruction set, this may eliminate the need for a locking mechanism (which would be necessary if the pointer and the boolean flag were separate).
  (Note: This practice is even possible to do safely in Java via java.util.concurrent.atomic.AtomicMarkableReference)

些什么？

推荐答案

我有时会投三分球为整数时，他们莫名其妙地需要一个哈希和的一部分。此外，我投他们整数做一些bitfiddling与他们在某些implemetnations它保证了指针总是有一个或两个备用比特离开，在那里我可以带左/右指针代替code AVL或RB树信息具有额外的成员。但是，这一切都是那么具体执行，我建议从来没有把它作为任何一种常见的解决方案。我又听见，有时危险的指针可以用这样的东西来实现。

I sometimes cast pointers to integers when they somehow need to be part of a hashsum. Also I cast them to integers to do some bitfiddling with them on certain implemetnations where it is guaranteed that pointers always have one or two spare bits left, where I can encode AVL or RB Tree information in the left/right pointers instead of having an additional member. But this is all so implementation specific that I recommend to never think about it as any kind of common solution. Also I heard that sometimes hazard pointers can be implemented with such a thing.

在某些情况下，我需要每个对象一个唯一的ID我一起例如通过服务器作为我的请求ID。根据上下文，当我需要节省一些内存，这是值得的，我用我的对象的地址作为这样一个id，通常都将它转换为整数。

In some situations I need a unique ID per object that I pass along to e.g. servers as my request id. Depending on the context when I need to save some memory, and it is worth it, I use the address of my object as such an id, and usually have to cast it to an integer.

在嵌入式系统的工作（如佳能相机，看到CHDK）经常有魔法addesses，所以（无效*）0xFFBC5235 或类似经常发现有太

When working with embedded systems (such as in canon cameras, see chdk) there are often magic addesses, so a (void*)0xFFBC5235 or similar is often found there too

编辑：

只是偶然（在我心中）在 pthread_self（）它返回的pthread_t通常是一个typedef无符号整数。虽然内部，它是一个指向某个线程结构，重presenting有问题的线程。一般来说它可能在其他地方使用的不透明句柄。

Just stumbled (in my mind) over pthread_self() which returns a pthread_t which is usually a typedef to an unsigned integer. Internally though it is a pointer to some thread struct, representing the thread in question. In general it might used elsewhere for an opaque handle.

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