swift default string interpolation for object [duplicate]

Question

I've made an class - call it Blah. It contains a string "value". So anytime someone makes a string out of it - I want it to appear as the value. So I try it:

let x = Blah( "hello world")
XCTAassertEqual("hello world", "\(x)")

out of the box - of course it doesn't work - I haven't told it how to do the rendering. So I do some googling and find that thing is "description" - so add this to Blah:

public var description = {get{return _value}}

and then the test becomes this:

let x = Blah( "hello world")
XCTAassertEqual("hello world", "\(x.description)")
XCTAassertEqual("hello world", "\(x)")

the first assert works, but the second is still giving me the whole object

so - more googling - I find that you can add custom string interpolations.. so I add this:

extension String.StringInterpolation {
    mutating func appendInterpolation(_ value: Blah) {
        appendLiteral( value.description)
    }
}

and that achieves... well... absolutely nothing. How do I change the way my objects are represented by string interpolation?

Answer 1

You were right about var description. However, you forgot to add the protocol CustomStringConvertible:

struct Blah {
    let value: String
}

extension Blah: CustomStringConvertible {
    var description: String { value }
}

let x = Blah(value: "Hello world")
print("Hello world: ", "\(x)")

Without the protocol, description is just a custom property.

Answer 2

I manage to get it to work.

class Blah {
    let value: String
    init(_ value: String) {
        self.value = value
    }
}

extension String.StringInterpolation {
    mutating func appendInterpolation(_ value: Blah) {
        appendLiteral(value.value)
    }
}

func testBlah() {
    let blah = Blah("Hello world")
    XCTAssertEqual("Hello world", "\(blah)")
}

The test passes.