Newbie programmer here using swift. How to change a struct to a class?

See more:

struct

Swift

I am using the Swift Programming language. Here is the code:

class CalculatorBrain9iOS{

    ***bunch of code***

    private var accumulator = 0.0
    private var pending: PendingBinaryOperationInfo?

    private struct PendingBinaryOperationInfo {
        var binaryFunction: (Double, Double) -> Double
        var firstOperand: Double
    }

    func ...(...) {
        if let operation = ... {
            switch operation {
            case ...
            case RandomFunction.ThisCase(let function):
                pending = PendingBinaryOperationInfo(binaryFunction: function,
                firstOperand: accumulator)
                
            }
        }
    }
 
   ***bunch of code***

}

What I have tried:

I want to change the private struct (PendingBinaryOperationInfo) to a class. It doesn't need inheritance so I figured I could just replace "private struct" with "class". But I keep getting initializer error messages on that line. Is it strictly because that new class is a nested class? Or are there other factors involved?

I guess my other big question here is: Aside from inheritance and value/reference, what other differences exist between classes and structs that makes it so that I can't just replace a struct with a class and still have the program run perfectly fine?

*PS: Sorry, buts it's not letting me indent the lines for some reason.