Which type of value can be compared in switch statement in Swift2

Question

I wonder which type of value can be compared in switch statement. The official document said:

Cases can match many different patterns, including interval matches, tuples, and casts to a specific type

Is there anything else? Can I compare class type in switch statement?

Suppose I hava a class A:

class A {
}
func == (lhs: A, rhs: A) -> Bool { return true }

Then I can check if two objects of class A are equal. But I still can't do like this:

var a1 = A(); var a2 = A()
switch a1 {
case a2: //do something 
}

Although we rarely write codes like these, I'm still very curious about how switch statement works in Swift.

jtbandes · Accepted Answer · 2015-12-17 09:14:35Z

3

As explained in Expression Patterns,

The expression represented by the expression pattern is compared with the value of an input expression using the Swift standard library ~= operator.

You can define func ~=(lhs: A, rhs: A) if you wish for your custom type to be used in a switch statement.

But I'd also recommend simply using the Equatable protocol, implementing ==, and then you can write if a1 == a2 { ... }.

In fact, the standard library provides

public func ~=<T : Equatable>(a: T, b: T) -> Bool

So if you conform to Equatable, you don't need to provide your own ~=.

edited Dec 17, 2015 at 9:14

answered Dec 17, 2015 at 8:46

jtbandes

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6 Comments

jtbandes Over a year ago

== is provided for NSObject subclasses, I believe. It uses isEqual(_:).

sevenkplus Over a year ago

It seems that I can write if a1 == a2 { ... } even if class A doesn't conform to Equatable protocol , and overload == operator is enough. In conclusion, do you mean any type which overloads their ~= operator can be compared in switch statement?

jtbandes Over a year ago

You can ⌘-click on the == operator to see where it's defined. Yes, ~= is what's used for switch statements.

sevenkplus Over a year ago

== is defined in Swift module. I am still not clear about if I have to conform to Equatable explicitly or just overload == operator is enough. Also, what do you mean by isEqual(_:), I think it is a method in Objective-C

jtbandes Over a year ago

If you are already providing == then I would recommend conforming to Equatable because you'll get access to various extensions such as SequenceType's split and startsWith etc. I also updated my answer — if you are Equatable, you don't need a custom ~=.

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