||= 在 Ruby 语言中是什么意思? [英] what does a ||= mean in Ruby language?

问题描述

<块引用>

可能的重复:
在 Ruby 中 ||= 是什么意思?

下面一行是什么意思?

a ||= {}一个 ||= 1

在 irb 中，对于上述两行，它总是以散列形式返回 a 的类.提前致谢.

解决方案

||= 是一个赋值运算符，它返回分配的值.a ||= b 等价于语句 a ||a = b 这意味着如果 a 被设置并且有一些真值，那么它保持不变，否则取b 的值.>

在您的示例中 a 只设置一次，这解释了您注意到的行为.

a ||= {}a ||= 1//a 仍然是 {}

我见过的典型用法是初始化静态变量，即.

class Foodef self.bar返回@bar ||= {}结尾结尾

值得一提的是，||= 是一个短路运算符.这意味着在 a ||= b 的情况下，只会有 a = b 的赋值.在 a 为非假的情况下，永远不会有 a = a 的赋值.这有点迂腐，但在某些(非常)边缘情况下很重要.

有关更多信息，阅读 ||= 主题和页面的最终列表.

Possible Duplicate:
What does ||= mean in Ruby?

what does the below line mean?

a ||= {} 
a ||= 1

in irb it always returns the class of a, as hash, for both the above lines. Thanks in advance.

解决方案

||= is an assignment operator, which returns the value assigned. a ||= b is equivalent to the statement a || a = b which means that if a is set and has some true value, then it remains the same, otherwise it takes the value of b.

In your example a is only ever set once, which explains the behaviour you've noticed.

a ||= {} 
a ||= 1 // a is still {}

Typical usage I've seen is to initialise static variables, ie.

class Foo
    def self.bar
        return @bar ||= {}
    end
end

EDIT:

It bears mentioning that ||= is a short-circuit operator. This means that it in the case of a ||= b there will only be an assignment of a = b. There will never be an assignment of a = a in the case that a is non-false. This is a little pedantic, but matters in some (very) edge cases.

For more information, read the definitive list of ||= threads and pages.

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