Cancelling a defer statement | Swift Pattern

I love the defer statement. It prevents so many error that could occur because one of the exits in a module may forget to close a file or release a resource etc.

I also do a fair amount of low-level programming and thus I get to write a lot of code that exits all over the place in case of an error, and has only one exit if everything went fine.

In order to use the defer statement, it need to release the resources on all the error exits, but not on the success exit.

So I would like to cancel a defer statement ... in a sense.

Of course that is not possible... is it?

No.

But there is a simple pattern that solves this problem neatly. It is so simple that it does not even need explaining:

func openFile(kind: String) -> UnsafeMutablePointer<FILE>? {

    

    var file = fopen("MyFile.txt", "r")

    

    var closeFile = true

    defer { if closeFile { fclose(file) } }

    

    if fileIsNotValid(file) { return nil }

    if fileDoesNotContainData(file, kind) { return nil }

    

    if fileDataOutOfDate(file) { return nil }

    

    // Prevent the deferred handler from closing the file

    closeFile = false

    return file

}

From Jay on the swift-evolution forum I received the following alternative:

func openFile(kind: String) -> UnsafeMutablePointer<FILE>? {

    

    var file = fopen("MyFile.txt", "r")

    

    var closeFile: (() -> Void)? = { fclose(file) }

    defer { closeFile?() }

    

    if fileIsNotValid(file) { return nil }

    if fileDoesNotContainsData(file, kind) { return nil }

    

    if fileDataOutOfDate(file) { return nil }

    

    // Prevent the deferred handler from closing the file

    closeFile = nil

    

    return file

}

Happy coding...

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