So far we have seen protocols that classes or structs can adopt to add more capabilities and perform more operations. However, the protocols we have discussed only work on specific data types. Consider the following protocol:

protocol ValuePrinting {
    var value: String { get }
    func printValue()
}

This protocol works only with a value of type String. If someone wanted to conform to ValuePrinting with a value of an Int, they would have to write a separate protocol.

The above problem could be solved using a generic protocol that can accept data of any type. The MessagePrinting protocol can be rewritten as follows:

protocol ValuePrinting {
    associatedtype T
    var value: T { get }
    func printValue()
}

In the above definition, we have used associatedtype T, which means it is left to the conforming class or struct to specify what that type should be. Therefore, the type of the value property will be inferred in the implementation of the conforming type.

We can implement ValuePrinting using a String:

struct StringPrinter: ValuePrinting {
    let value: String

    func printValue() {
        print("My string value is \(value)")
    }
}

And we can also implement ValuePrinting using a Double:

struct DoublePrinter: ValuePrinting {
    let value: Double

    func printValue() {
        print("My double value is \(value)")
    }
}