存在非短路逻辑运算符的原因 [英] Reason for the existence of non-short-circuit logical operators

问题描述

与布尔操作数一起使用时，& 和 | 每逻辑运算符rel =nofollow noreferrer> JLS的第15.22.2节。然而，与&& 和 || 不同，这些不会短路;他们总是评估双方。我有一个愚蠢的问题：为什么效率较低的非短路逻辑运算符（& ， | ）当我们拥有效率更高的短路逻辑运算符时（&& ， || ）？我的意思是，与短路逻辑运算符相比，非短路逻辑运算符的实际用途是什么？换句话说，总是通过使用非短路逻辑运算符来评估双方的用法是什么？

When used with boolean operands, & and | become logical operators per Section 15.22.2 of the JLS. Unlike && and ||, however, these don't short-circuit; they always evaluate both sides. I have a silly question: Why are the less-efficient non-short-circuit logical operators (&, |) still there, when we have the more-efficient short-circuit logical operators (&&, ||)? I mean, what is the real usage of the non-short-circuit logical operators, as opposed to with the short-circuit logical operators? In other words, what is the usage of always evaluating both sides by using the non-short-circuit logical operators?

推荐答案

更新的答案：

道歉，我在你的问题中错过了逻辑这个词，即使是那里。 （我冒昧地通过编辑强调它。）

考虑你想要任何副作用的情况无论左侧表达式是否计算 true 或 false ，都会发生。例如，对比：

Consider the case where you want any side-effects to always occur, regardless of whether the left-hand expression evaluates true or false. E.g., contrast:

if (foo() & bar()) {
    // Only call this if both operations returned true
}

with

if (foo() && bar()) {
    // Only call this if both operations returned true
}

我们假设 foo 和吧无论 foo 是否返回 true 都会产生想要的效果c $ c>或 false 。在上面的第一个中，我知道 bar 将始终被调用并产生效果。当然，在后者中， bar 可能会被调用，也可能不被调用。如果我们没有非短路版本，我们必须使用临时变量：

Let's assume both foo and bar have effects that we want to have happen regardless of whether foo returns true or false. In the first one above, I know that bar will always get called and have its effect. In the latter, of course, bar may or may not get called. If we didn't have the non-short-circuit version, we'd have to use temporary variables:

boolean fooResult, barResult;
fooResult = foo();
barResult = bar();
if (fooResult && barResult) {
    // ...
}

你可能会争辩（我可能会）你应该无论如何，因为它太容易误读 if（foo（）& bar （）），但我们去了，这是一个非短路版本的实用理由。

You might argue (I probably would) that you should do that anyway, because it's way too easy to misread if (foo() & bar()), but there we go, a pragmatic reason for having non-short-circuit versions.

原始答案：

您如何建议& （或 | ）是一个短路的运营商？使用&& 和 || ，这是有道理的，因为你正在处理布尔条件：它们可以是无论是真是假，都没有灰色阴影。但是& 和 | 处理比特而不是布尔值。结果是一个数字。我的意思是，如果左侧是 0 ，我猜& 无法评估右侧，并且类似地， | 无法评估它，如果左侧是全部位 - 无论类型是什么，但我没有看到制作一个每个运算符的边缘情况显着（与254或更多其他情况相比）。

How would you propose & (or |) be a short-circuited operator? With && and ||, it makes sense because you're dealing with boolean conditions: They can be true or false, there are no shades of grey. But & and | deal with bits, not booleans. The result is a number. I mean, I guess & could not evaluate the right-hand side if the left-hand side were 0, and similarly | could not evaluate it if the left-hand side were all-bits-on for whatever the type was, but I don't see much point to making the one edge case of each operator significant (as compared to the 254 or more other cases).

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