?? Swift中的运算符 [英] ?? operator in Swift

问题描述

在"Swift编程语言"一书(第599页)中，我遇到了使我感到困惑的这段代码.它是这样的:

In the "The Swift Programming Language" book (page 599), I came across this code snippet that kind of confused me. It went like this:

func buyFavoriteSnack(person:String) throws {
    let snackName = favoriteSnacks[person] ?? "Candy Bar"
    try vend(itemName:snackName)
}

它的解释是:

buyFavoriteSnack(_ :)函数查找给定人的最爱小吃，并尝试为他们购买.如果他们没有列出最喜欢的小吃，它会尝试购买糖果.如果他们...

The buyFavoriteSnack(_:) function looks up the given person's favorite snack and tries to buy it for them. If they don't have a favorite snack listed, it tries to buy a candy bar. If they...

此说明如何映射到"??"给定代码中的运算符.我们何时/应该在自己的代码中使用此语法?

How can this explanation map to the "??" operator in the code given. When should/can we use this syntax in our own code?

推荐答案

它是零合并运算符"(也称为默认运算符").除非a是nil，否则a ?? b是a的值(即a!)，在这种情况下，它会产生b. IE.如果缺少favouriteSnacks[person]，则代之以赋值"Candy Bar".

It is "nil coalescing operator" (also called "default operator"). a ?? b is value of a (i.e. a!), unless a is nil, in which case it yields b. I.e. if favouriteSnacks[person] is missing, return assign "Candy Bar" in its stead.

编辑 从技术上讲，可以解释为:(摘自下面的Badar Al-Rasheed的回答)

EDIT Technically can be interpreted as: (From Badar Al-Rasheed's Answer below)

let something = a != nil ? a! : b

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