如何定义具有多个特定自定义类型的变量？ （在斯威夫特中）

Question

Hi guys as you may know sometimes, it would be so handy and clean if we could define a variable which can store multiple variable types. this operation is easily possible if we use "Any" instead of our type name. for example:

var a : Any     // defines a variable with ability to store all types except a nil
var b : Any?    // defines a variable with ability to store all types including nil
var c : String  // a variable storing only and only a String value
var d : String? // a variable such as "c" , but also includes nil too

in the first two ones we can store all {Int, String, Float & etc.}. also in the third and fourth one we can store String values while we can't store others such as "Int" or "Float". but what if in some cases we would like to have a variable which can store custom Types? for example i want a variable which can store an "Int" value, can store a "String" one, but can not store "Float"s?

var e : only(String and Int)
// some code like above, or even below :
var f : Any but not Float

is there any idea? is there any solution please?

thanks dudes

Answer 1

Currently there is no way to achieve this functionality. However, a closer, and in my opinion, a better way would be to use Any type and then use a guard or if let while casting the value to your desired type. An example of such can be seen below:

Example

let a: Any    //e.g. you only want to store Int and String in this variable
if let _ = a as? Int {
    //a is of type Int
} else if let _ = a as? String {
    //a is of type String
}

Answer 2

Seems like you need to use protocols .

Something like:

protocol CustomType {
    // your definitions ...
}

extension String: CustomType {}
extension Int: CustomType {}

let customType1: CustomType = "string"
print(customType1)
let customType2: CustomType = 0
print(customType2)

// Error: Value of type 'Double' does not conform to specified type 'CustomType'
// let customType3: CustomType = 0.0

In this case, values of CustomType type will only accept String and Int types (because of the protocol conformance).

Answer 3

For most cases, when you want to store multiple types in a single variable or in a collection you would use an enum with associated values:

enum StringOrInt {
    case string(String)
    case int(Int)
}

var e: StringOrInt = .string("Hello World")
var f: StringOrInt = .int(42)
var g: [StringOrInt] = [e, f]