问题描述
假设 A ， B ， A ，和 b 都是变量， A ， b的地址， A 和 b 都是不同的。然后，以下code：
Suppose A, B, a, and b are all variables, and the addresses of A, B, a, and b are all different. Then, for the following code:
A = a;
B = b;
不要C和C ++标准明确要求 A = A 严格之前执行B = ？鉴于 A ， B ， A 的地址，和 b 都是不同的，都允许编译器交换两个语句的执行顺序为了某种目的，如优化？
Do the C and C++ standard explicitly require A=a be strictly executed before B=b? Given that the addresses of A, B, a, and b are all different, are compilers allowed to swap the execution sequence of two statements for some purpose such as optimization?
如果回答我的问题是，在C和C ++不同，我想知道这两个。
If the answer to my question is different in C and C++, I would like to know both.
编辑：这个问题的背景如下。在板游戏AI设计，对于优化的人使用时，其正确性强烈依赖于执行顺序，如果我们不加挥发性限制。
The background of the question is the following. In board game AI design, for optimization people use lock-less shared-hash table, whose correctness strongly depends on the execution order if we do not add volatile restriction.
推荐答案
这两种标准都允许作为不改变观察到的行为进行乱序执行这些指令，那么长。这就是所谓的AS-if规则：
Both standards allow for these instructions to be performed out of order, so long as that does not change observable behaviour. This is known as the as-if rule:
- What exactly is the "as-if" rule?
- http://en.cppreference.com/w/cpp/language/as_if
请注意，由于在评论中指出，什么是观察的行为的意思是定义的行为程序的观察行为。如果你的程序有不确定的行为，那么编译器是从推理的借口。
Note that as is pointed out in the comments, what is meant by "observable behaviour" is the observable behaviour of a program with defined behaviour. If your program has undefined behaviour, then the compiler is excused from reasoning about that.
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