短路评价和副作用

本文介绍了短路评价和副作用的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

OK，我有点不好意思问这个问题，但我只是想确保...

据了解，C使用短路评价布尔前pressions：

INT C = 0;如果（C＆安培;＆安培; FUNC（C））{/ *什么... * /}

在这个例子中 FUNC（C）不叫，因为 C 计算结果为 0 。但如何对更复杂的例子，其中比较副作用会改变变量被下一个比较？像这样的：

INT℃; / *这甚至不是初始化... * /如果（canInitWithSomeValue（和C）及和C ==东西）{} /*...*/

功能 canInitWithSomeValue 返回给定指针在成功的情况下真实的变化值。它是保证后续的比较（ç== SOMETHING 在这个例子中）使用值由设置canInitWithSomeValue（和C）？

无论编译器优化多么沉重使用？

解决方案   

它是保证后续的对比（在本例C ==东西）使用值由canInitWithSomeValue设置？（和C）

是的。因为有一个序列点

  

的左侧和右侧的操作数的评价之间的＆放大器;＆安培; （逻辑与）， || （逻辑OR）和逗号操作符。例如，在EX pression * P ++ = 0＆安培;！＆安培; * Q ++！= 0 ，子恩pression * P ++的所有副作用！= 0的任何企图访问q前完成。

  

一个序列点定义在它被保证previous评估所有的副作用将已被执行的计算机程序的执行的任何点，并且还没有被执行无副作用从后续评估

OK, I'm a little embarassed to ask this question, but I just want to be sure...

It is known that C uses short circuit evaluation in boolean expressions:

int c = 0; if (c && func(c)) { /* whatever... */ }

In that example func(c) is not called because c evaluates to 0. But how about more sophisticated example where side effects of comparison would change the variable being compared next? Like this:

int c; /* this is not even initialized... */ if (canInitWithSomeValue(&c) && c == SOMETHING) { /*...*/ }

Function canInitWithSomeValue returns true and changes value at given pointer in case of success. Is it guaranteed that subsequent comparisons (c == SOMETHING in this example) uses value set by canInitWithSomeValue(&c)?

No matter how heavy optimizations the compiler uses?

解决方案

Is it guaranteed that subsequent comparisons (c == SOMETHING in this example) uses value set by canInitWithSomeValue(&c)?

Yes. Because there is a sequence point

Between evaluation of the left and right operands of the && (logical AND), || (logical OR), and comma operators. For example, in the expression *p++ != 0 && *q++ != 0, all side effects of the sub-expression *p++ != 0 are completed before any attempt to access q.

A sequence point defines any point in a computer program's execution at which it is guaranteed that all side effects of previous evaluations will have been performed, and no side effects from subsequent evaluations have yet been performed.