Swift 如何自定义运算符？

Swift 中许多有的运算符，可以方便做一些运算操作。 例如:

• 溢出运算符 &+= &-= ...
• 范围运算符 ..< ...
• 空合运算符 ?? 等等

那么，这些运算符的如何进行定义、优先级大小和结合性如何设置等等，让我们一起来了解一下吧！

如何定义运算符？

/// 前缀
prefix operator ++

/// 中缀
infix operator **

/// 后缀
postfix operator --

运算符的优先级

优先级组

//===----------------------------------------------------------------------===//
// Standard precedence groups
//===----------------------------------------------------------------------===//
precedencegroup AssignmentPrecedence {
  assignment: true
  associativity: right
}
precedencegroup FunctionArrowPrecedence {
  associativity: right
  higherThan: AssignmentPrecedence
}
precedencegroup TernaryPrecedence {
  associativity: right
  higherThan: FunctionArrowPrecedence
}
precedencegroup DefaultPrecedence {
  higherThan: TernaryPrecedence
}
precedencegroup LogicalDisjunctionPrecedence {
  associativity: left
  higherThan: TernaryPrecedence
}
precedencegroup LogicalConjunctionPrecedence {
  associativity: left
  higherThan: LogicalDisjunctionPrecedence
}
precedencegroup ComparisonPrecedence {
  higherThan: LogicalConjunctionPrecedence
}
precedencegroup NilCoalescingPrecedence {
  associativity: right
  higherThan: ComparisonPrecedence
}
precedencegroup CastingPrecedence {
  higherThan: NilCoalescingPrecedence
}
precedencegroup RangeFormationPrecedence {
  higherThan: CastingPrecedence
}
precedencegroup AdditionPrecedence {
  associativity: left
  higherThan: RangeFormationPrecedence
}
precedencegroup MultiplicationPrecedence {
  associativity: left
  higherThan: AdditionPrecedence
}
precedencegroup BitwiseShiftPrecedence {
  higherThan: MultiplicationPrecedence
}

以上是 swift 源码中关于标准优先级组的定义。当然，我们也可以查看官方的文档了解运算符的优先级组。下表展示了优先级由低到高的排列

优先级

优先级(由低到高)	结合性	运算符
AssignmentPrecedence	right	*= &*= /= %= += &+= -= &-= <<= &<<= >>= &>>= &= ^= |=
FunctionArrowPrecedence	right	~>
TernaryPrecedence	right	?:
DefaultPrecedence	none
LogicalDisjunctionPrecedence	left	||
LogicalConjunctionPrecedence	left	&&
ComparisonPrecedence	none	< <= > >= == != === !== ~=
NilCoalescingPrecedence	right	??
CastingPrecedence	none	is as as! as?
RangeFormationPrecedence	none	... ..<
AdditionPrecedence	left	+ &+ - &- | ^
MultiplicationPrecedence	left	* &* / % &
BitwiseShiftPrecedence	none	<< &<< >> &>>

• 中缀运算符默认的优先级别为 DefaultPrecedence。
• 中缀运算符默认的结合性为 none。
• DefaultPrecedence 的优先级高于 TernaryPrecedence。

如何自定义运算符？

// 1
precedencegroup PowerPrecedence {
    associativity: left
    higherThan: MultiplicationPrecedence
}

// 2
infix operator ** : PowerPrecedence

// 3
extension Int {
    static func ** (lhs: Int, rhs: Int) -> Int {
        return Int(pow(Double(lhs), Double(rhs)))
    }
}

1. 定义优先级组 PowerPrecedence,设置结合性为 left,优先级大于 MultiplicationPrecedence
2. 定义运算符 **,设置优先级别为 PowerPrecedence
3. 扩展 Int，添加**运算。