如何评估“if（A&& B）”语句？ [英] How an 'if (A && B)' statement is evaluated?

问题描述

if( (A) && (B) )
{
  //do something
}
else
  //do something else

问题是，如果A是假。 B甚至会得到评估？

The question is, would the statement immediately break to else if A was FALSE. Would B even get evaluated?

在这种情况下，当数组实际为空且有零个元素时，B检查数组索引的有效性为array [0]。因此，抛出一个segfault，因为我们试图访问超出数组的边界。

I ask this in the case that B checking the validity of an array index say array[0] when the array is actually empty and has zero elements. Therefore throwing a segfault because we are trying to access something that is out of bounds of the array. Specifically

if( (array.GetElements() > 0) && (array[0]))
  array[0]->doSomething();
else
  //do nothing and return

[0]实际上得到评估，因为它在没有第一个检查到'&&'左侧的segfaults。 Precedence告诉我，左侧肯定会优先，但它不告诉我，如果左侧是FALSE它不会评估右侧。

This may be dangerous if array[0] actually gets evaluated because it segfaults without the first check to the left of the '&&'. Precedence tells me that the left side will definitely take precedence but it doesn't tell me that it won't evaluate the right side if the left is FALSE.

推荐答案

在C和C ++中，&& 和 || 电路。这意味着它们只在需要时评估参数。如果&& 的第一个参数为false，或者第一个参数为 ||

In C and C++, the && and || operators "short-circuit". That means that they only evaluate a parameter if required. If the first parameter to && is false, or the first to || is true, the rest will not be evaluated.

您发布的代码是安全的，但我怀疑为什么要包含一个空的 else 块。

The code you posted is safe, though I question why you'd include an empty else block.

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