The defer statement | Swift gotcha

I love the defer statement. Use it all the time...

(Note: The defer statement allows you to execute a piece of code just before the scope the statement is in closes)

However today I ran into an inexplicable problem where a file read operation would always fail. And as so often I simply could not see the error until I stepped through. The error turned out to be quite simple: a defer statement at the wrong place.

This was the offending code:

let path = "/Users/Home/Me/Desktop/file.txt"

let file = fopen(path, "r")

if file == nil {

    print("Error opening file at \(path)")

} else {

    defer { fclose(file) }

}

let result = read(from: file)

...

Do you spot the error?

Right! the defer statement is executed immediately because the end-of-scope is the "}" closing the "else" part.

So the file was already closed when the read operation was called.
The solution is obvious:

let path = "/Users/Home/Me/Desktop/file.txt"

let file = fopen(path, "r")

if file == nil {

    print("Error opening file at \(path)")

}

defer { fclose(file) }

let result = read(from: file)

...

The rule of thumb is clear: Always place the defer stement at the same "level" or "indent" as the variable it refers to. Which -to be clear- simply means "the same scope".

Happy coding...

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