How to use do-catch functions in Swift-CodePudding

I am a novice with Swift and iOS development and I have come across a weird situation.

I am trying to use a do-catch statement to prevent an error, but it seems like the do-catch is doing nothing at all.

Here's my code:

extension String {
    mutating func insert(string: String, ind: Int) {
        do { 
            try self.insert(contentsOf: string, at: self.index(self.startIndex, offsetBy: ind))
        // This do-catch is to make sure nothing happens if there is no √ in the equation.
        } catch {
          
        }
    }
}

The 'try' statement does not seem to be doing anything either! I have searched and searched online forums and have not come up with anything useful.

I know Java well, and you can always catch a specific error like:

try {

} catch NameOfException {

}

But I have found nothing the like in Swift. The error code that is returned is this:

Swift/StringCharacterView.swift:60: Fatal error: String index is out of bounds
2022-03-19 17:37:04.234797-0600 iOS-App[18937:2984774] Swift/StringCharacterView.swift:60: Fatal error: String index is out of bounds
(lldb)

I interpret this to mean that the String index is clearly out of bounds, but I cannot find a method (like Java) to catch this kind of expression, or if I can even handle errors like Java at all. But I want to catch it so that my app does not crash.

I'm using a function to try and locate all instances of '√' inside of a String; myString.insert() ind is the Index to insert the provided String value.

CodePudding user response：

You can just add throws to your method signature and throw a custom error. I would also make the method generic and extend StringProtocol instead of String to support Substring as well. Note that constraining to RangeReplaceableCollection is required to be able to use mutating func insert<S>(contentsOf newElements: S, at i: Index) where S: Collection, Element == S.Element:

extension String {
    enum Error: Swift.Error {
        case invalidIndexDistance
    }
}

extension StringProtocol where Self: RangeReplaceableCollection {
    mutating func insert<S: StringProtocol>(contentsOf string: S, at distance: Int) throws {
        guard let index = self.index(startIndex, offsetBy: distance, limitedBy: endIndex) else {
            throw String.Error.invalidIndexDistance
        }
        insert(contentsOf: string, at: index)
    }
}

var substring = "abcde".dropFirst()
do {
    try substring.insert(contentsOf: "fgh", at: 2)
    substring  // "bcfghde"
} catch {
    print(error)
}

CodePudding user response：

I have found a relatively dirty way to fix the unhandled error, like @Alexander said.

extension String {
    mutating func insert(sourceString: String, string: String, ind: Int) {
        do {
            if ind > sourceString.count {
              
            } else {
                try insert(contentsOf: string, at: index(startIndex, offsetBy: ind))
            }
        // This do-catch is to make sure nothing happens if there is no √ in the equation.
        } catch GenericError.StringException {
          
        }
    }
}

Thanks for your help guys!