Playground demonstrates how can you properly add String conversion to your own classes. See http://blog.redgreenrefactor.eu/post/139107029424/string-conversions-in-swift.

SwiftStringConversion.playground

//: Playground - noun: a place where people can play

class MyClass: CustomStringConvertible {
    let counter: Int
    
    var description: String {
        return "{ counter: \(self.counter) }"
    }
    
    init(withCounter: Int) {
        self.counter = withCounter
    }
}

//extension String {
//    init(_ myClass: MyClass) {
//        self = "{ counter: \(myClass.counter) }"
//    }
//}

let str = String(MyClass(withCounter: 25))

let str2 = "\(MyClass(withCounter:25))"

let str3 = "\(String(MyClass(withCounter:25)))"