apache poi从右到左(right to left with apache poi)

 在C ++标准中，定义了一个种族： 
 
 
 
 1.10 / 4：如果其中一个表达式评估修改了内存位置，而另一个表达式评估访问或修改了相同的内存位置，则冲突。 
 
 
 1.10 / 21：如果一个程序的执行包含一个数据竞争，如果它包含两个不同线程中的冲突动作，其中至少有一个不是原子的，而且两个都不会发生在另一个线程之前。 任何这样的数据竞争都会导致未定义的行为。 
 
 
 假设你有多个线程运行相同的代码，由于func()总是返回0（你的声明），所有线程都不能改变x的内容。 而且，y是由线程执行的函数的局部变量，所以它不被共享。 因此，在这种情况下不会出现竞争状况。 
 
 编译器不允许进行与第二个片段相对应的转换，因为： 
 
 
 
 1.10 / 22：编译器转换将分配引入可能共享的内存位置，这个内存位置不会被抽象机器修改，这个标准通常会被排除在外，因为这样的分配可能会在另一个抽象机器执行不会遇到数据竞争。 
 
 
 但是如果你自己写了这段代码，在上面解释的条件下可能遇到赛车条件，因为x不是原子的，并且在一个线程（ temp=x ）中可以有读取访问权限，在另一个线程中可以写入访问权限（ x=0或in另一个线程的默认部分（ x=temp ） 

In the C++ standard, a race is defined: 
 
 
 
1.10/4: Two expression evaluations conflict if one of them modifies a memory location and the other one accesses or modifies the same memory location.
 
 
1.10/21: The execution of a program contains a data race if it contains two conflicting actions in different threads, at least one of which is not atomic, and neither happens before the other. Any such data race results in undefined behavior.
 
 
Supposing that you have several threads running the same code, due to the fact that func() would always return 0 (your claim), none of the threads could change the content of x. Furthermore, y is a local variable of a function executed by a thread, so it is not shared. Hence, no race condition could occur in this scenario. 
 
The compiler is not allowed to make the transformations corresponding to the second snippet because: 
 
 
 
1.10/22: Compiler transformations that introduce assignments to a potentially shared memory location that would not be modified by the abstract machine are generally precluded by this standard, since such an assignment might overwrite another assignment by a different thread in cases in which an abstract machine execution would not have encountered a data race.
 
 
But if yourself write the snippet, under the conditions explained above might encounter racing conditions since the x is not atomic, and there could be read access in one thread (temp=x) and write access in the other (either x=0 or in the default section of the other thread (x=temp)