有没有办法在Swift中为`struct`自动定义compare(`==`)函数？

让我们假设我们在 Swift中有一个非常大的结构：

struct SuperStruct {
    var field1: Int = 0
    var field2: String = ""
    // lots of lines...
    var field512: Float = 0.0
}

..然后我们需要实现Equatable协议：

extension SuperStruct: Equatable {
}

func ==(lhs: SuperStruct, rhs: SuperStruct) -> Bool {
    return
        lhs.field1 == rhs.field1 &&
        lhs.field2 == rhs.field2 &&
        // lots of lines...
        lhs.field512 == rhs.field512
}

……我们需要编写许多愚蠢的代码行.
有没有办法“问”编译器为我们“做”它？

以下答案显示了一种可能的解决方案可能不是推荐的(但可能对此问题的未来读者感兴趣).

如果你有大量的属性都属于有限数量的不同类型,你可以使用结构实例的镜像并遍历结构的属性;每次尝试转换为您知道属性的不同类型.

在观看了以下WWDC 2015会议之后,我已经编辑了之前的答案(对于我认为非常整洁的东西)(感谢Leo Dabus！)：

> WWDC 2015 session 408.推荐.

我将在这个答案的底部留下最初的答案,因为它显示了一种替代的,不那么面向协议的方法,以利用这个Mirror解决方案.

镜子&面向协议的解决方案：

/* Let a heterogeneous protocol act as "pseudo-generic" type
   for the different (property) types in 'SuperStruct'         */
protocol MyGenericType {
    func isEqualTo(other: MyGenericType) -> Bool
}
extension MyGenericType where Self : Equatable {
    func isEqualTo(other: MyGenericType) -> Bool {
        if let o = other as? Self { return self == o }
        return false
    }
}

/* Extend types that appear in 'SuperStruct' to MyGenericType  */
extension Int : MyGenericType {}
extension String : MyGenericType {}
extension Float : MyGenericType {}
    // ...

/* Finally, 'SuperStruct' conformance to Equatable */
func ==(lhs: SuperStruct, rhs: SuperStruct) -> Bool {

    let mLhs = Mirror(reflecting: lhs).children.filter { $0.label != nil }
    let mRhs = Mirror(reflecting: rhs).children.filter { $0.label != nil }

    for i in 0..<mLhs.count {
        guard let valLhs = mLhs[i].value as? MyGenericType, valRhs = mRhs[i].value as? MyGenericType else {
            print("Invalid: Properties 'lhs.\(mLhs[i].label!)' and/or 'rhs.\(mRhs[i].label!)' are not of 'MyGenericType' types.")
            return false
        }
        if !valLhs.isEqualTo(valRhs) {
            return false
        }
    }
    return true
}

用法示例：

/* Example */
var a = SuperStruct()
var b = SuperStruct()
a == b // true
a.field1 = 2
a == b // false
b.field1 = 2
b.field2 = "Foo"
a.field2 = "Foo"
a == b // true

Previous Mirror解决方案：

/* 'SuperStruct' conformance to Equatable */
func ==(lhs: SuperStruct, rhs: SuperStruct) -> Bool {

    let mLhs = Mirror(reflecting: lhs).children.filter { $0.label != nil }
    let mRhs = Mirror(reflecting: rhs).children.filter { $0.label != nil }

    for i in 0..<mLhs.count {
        switch mLhs[i].value {
        case let valLhs as Int:
            guard let valRhs = mRhs[i].value as? Int where valRhs == valLhs else {
                return false
            }
        case let valLhs as String:
            guard let valRhs = mRhs[i].value as? String where valRhs == valLhs else {
                return false
            }
        case let valLhs as Float:
            guard let valRhs = mRhs[i].value as? Float where valRhs == valLhs else {
                return false
            }
            /* ... extend with one case for each type
            that appear in 'SuperStruct'  */
        case _ : return false
        }
    }
    return true
}

用法示例：

/* Example */
var a = SuperStruct()
var b = SuperStruct()
a == b // true
a.field1 = 2
a == b // false
b.field1 = 2
b.field2 = "Foo"
a.field2 = "Foo"
a == b // true