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Post: Swift : Changing the Value of Struct in an Array

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Swift : Changing the Value of Struct in an Array

Published 12:11 Nov 20, 2015.

Created by @ezra. Categorized in #Programming, and tagged as #iOS.

Source format: Markdown

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今天帮朋友看了这样一个问题:

import UIKit

struct A {
    var a:Bool
}

struct B {
    var b : String
    var ba : A
}

struct C {
    var c : [B]
}

var cc = C(c: [
                B(b: "1", ba: A(a: true)),
                B(b: "2", ba: A(a: true)),
                B(b: "3", ba: A(a: true)),
                B(b: "4", ba: A(a: true)),
                ]
            )

for var b in cc.c {
    b.b = "22"
}
print(cc)

输出结果为:

C(c: [B(b: "1", ba: A(a: true)), B(b: "2", ba: A(a: true)), B(b: "3", ba: A(a: true)), B(b: "4", ba: A(a: true))])

很显然这个企图修改数组中结构体内容的操作失败了,为什么呢?我在代码中 for 部分添加了两个输出:

for var b in cc.c {
    b.b = "22"
    print(b)
    print(cc.c)
}

看一下输出结果:

B(b: "22", ba: A(a: true))
[B(b: "1", ba: A(a: true)), B(b: "2", ba: A(a: true)), B(b: "3", ba: A(a: true)), B(b: "4", ba: A(a: true))]
B(b: "22", ba: A(a: true))
[B(b: "1", ba: A(a: true)), B(b: "2", ba: A(a: true)), B(b: "3", ba: A(a: true)), B(b: "4", ba: A(a: true))]
B(b: "22", ba: A(a: true))
[B(b: "1", ba: A(a: true)), B(b: "2", ba: A(a: true)), B(b: "3", ba: A(a: true)), B(b: "4", ba: A(a: true))]
B(b: "22", ba: A(a: true))
[B(b: "1", ba: A(a: true)), B(b: "2", ba: A(a: true)), B(b: "3", ba: A(a: true)), B(b: "4", ba: A(a: true))]

这样问题就清晰了,所以我们换一种方式来试一试:

for var i = 0; i < cc.c.count; i++ {
    cc.c[i].b = "11"
}
print(cc)

再看一下输出结果,成功~

C(c: [B(b: "11", ba: A(a: true)), B(b: "11", ba: A(a: true)), B(b: "11", ba: A(a: true)), B(b: "11", ba: A(a: true))])

也就是说,我们通过 for var b in cc.c 遍历结构体时,我们的操作仅仅局限于 var b,对 cc.c 中的 b : B 没有影响,这是因为 var b 并不是对 cc.c 数组中元素的引用,而是一份拷贝,与原有元素不存在直接联系。

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