Multiple protocol and class inheritance confusion, Swift

Question

So I know a class can only inherit a single class but can inherit multiple protocols. I am looking at some code here and confused as to why swift is throwing me an error.

protocol PaymentViewModelConfigurable: ViewModelWithResult {
    
}

class ViewModelWithResult {
    func printWithResultClass() {
        print("In View Model with result class")
    }
}

class PaymentViewModel: PaymentViewModelConfigurable {
    
    
}


class MainOne {
    let viewModel: PaymentViewModelConfigurable = PaymentViewModel()
}

So I would assume this is ok because my PaymentViewModel class inherits a protocol and that protocol inherits from a class.

But if I change the logic to this on my ViewModel to inherit the protocol & the class, its fine

protocol PaymentViewModelConfigurable: ViewModelWithResult {
    func payments()
}

class ViewModelWithResult {
    func printWithResultClass() {
        print("In View Model with result class")
    }
}

class PaymentViewModel: ViewModelWithResult, PaymentViewModelConfigurable {
    func payments() {
        print("Payments")
    }
}


class MainOne {
    let viewModel: PaymentViewModelConfigurable = PaymentViewModel()
    
    init() {
        viewModel.payments()
    }
}

These are the errors that come up:

'PaymentViewModelConfigurable' requires that 'PaymentViewModel' inherit from 'ViewModelWithResult'

Type 'PaymentViewModel' does not conform to protocol 'PaymentViewModelConfigurable'

And if you are going to ask about errors, please show them as well. (I know they were in the original image, but lost in the translation to text.) — Scott Hunter, Jul 06 '23 at 19:27
@SimonMcNeil Up to you! You can either put them as comment inline to where the errors appear, or paste them in a separate snippet below the rest of your code — Alexander, Jul 06 '23 at 19:30

Alexander · Accepted Answer · 2023-07-06T19:34:09.093

So I know a class can only inherit a single class but can inherit multiple protocols.

Not quite. A class can conform to multiple protocols. It might seem like a pedantic correction of wording, but it's actually core to the issue here. The conformance and inheritance features are related and interconnected, but distinct. Similarly:

and that protocol inherits from a class.

It doesn't.

Everything after the : on a declaration of a protocol is not a list of superclasses and protocol conformances, like with classes. It's a list of requirements of that protocol. So this declaration:

protocol PaymentViewModelConfigurable: ViewModelWithResult {
    func payments()
}

Is saying: "PaymentViewModelConfigurable is a protocol that can be conformed to anything that's a subtype of ViewModelWithResult and has a func payments() method. It's as if you had written:

protocol PaymentViewModelConfigurable where Self: ViewModelWithResult {
    func payments()
}

The error messages spell this out quite explicitly:

Untitled 4.swift:11:7: error: 'PaymentViewModelConfigurable' requires that 'PaymentViewModel' inherit from 'ViewModelWithResult'
class PaymentViewModel: PaymentViewModelConfigurable {
      ^
Untitled 4.swift:11:7: note: requirement specified as 'Self' : 'ViewModelWithResult' [with Self = PaymentViewModel]
class PaymentViewModel: PaymentViewModelConfigurable {
      ^

putting the `where Self:` instead helped me understand it better. Thanks! — Simon McNeil, Jul 06 '23 at 20:07

Multiple protocol and class inheritance confusion, Swift

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