Swift: 具有多个模式的 Switch case 无法绑定到变量

Question

在官方 Swift Programming Language 指南中，关于 switch case 是这样说的： “...如果案例包含与控制表达式匹配的多个模式，这些模式中的 none 可以包含常量或变量绑定。”

包含多个模式是什么意思？

Answer 1

意思是多模式的case标签不能声明变量。

这是允许的：

let somePoint = (1, 1)
switch somePoint {
// Case with multiple patterns without binding
case (0, _),
     (_, 0):
    println("(\(somePoint.0), \(somePoint.1)) is on an axis")
default:
    println("(\(somePoint.0), \(somePoint.1)) is not of an axis")
}

这也是允许的：

let somePoint = (1, 1)
switch somePoint {
// Case with single patterns with binding
case (0, let y):
    println("(0, \(y)) is on an axis")
case (let x, 0):
    println("(\(x), 0) is on an axis")
default:
    println("(\(somePoint.0), \(somePoint.1)) is not of an axis")
}

但是，这是被禁止的：

let somePoint = (1, 1)
switch somePoint {
// Case with multiple patterns that have bindings
case (0, let y),
     (let x, 0):
    println("(\(x), \(y)) is on an axis")
default:
    println("(\(somePoint.0), \(somePoint.1)) is not of an axis")
}

以上产生错误：

error: 'case' labels with multiple patterns cannot declare variables

Swift: 具有多个模式的 Switch case 无法绑定到变量

Swift: Switch case with multiple patterns cannot bind to variable

switch-statement

swift