Issue Using Swift's withUnsafeMutableBytes Wrapper

Question

I'm trying to write an extension to Data that allows me to extract it in various casts.

I'm running into a strange problem that I can't quite figure out.

Before I get into generics, I'm trying out fixed data types, and I created this method:

func intValueFromData(_ inData: Data) -> Int64? {
    var number = Int64(0)
    let len = Swift.min(MemoryLayout<Int64>.size, inData.count)
    _ = withUnsafeMutableBytes(of: &number) {
        inData.copyBytes(to: $0, from: 0..<len)
    }
    
    return number
}

That works. If I do this:

var int_64 = Int64(12345)

var data = Data(bytes: &int_64, count: MemoryLayout<Int64>.size)

let fetched = intValueFromData(data)

fetched becomes "12345" as an Int64.

However, when I try to embed the same method into the Data type, like so:

extension Data {
    mutating func intValueFromData() -> Int64? {
        var number = Int64(0)
        let len = Swift.min(MemoryLayout<Int64>.size, self.count)
        _ = withUnsafeMutableBytes(of: &number) {
            self.copyBytes(to: $0, from: 0..<len)
        }
        
        return number
    }
}

I get a compile-time error that says that "withUnsafeMutableBytes(of: &number)" is not supposed to have an argument.

The last time that I encountered something like this, it turned out that Apple explicitly blocked a functionality, but neglected to tell us in a straightforward manner.

I am not an expert at this kind of thing, but I am wondering if anyone could shed any light on why withUnsafeMutableBytes(of: &number) behaves differently inside the extension.

Answer 1

In the context of Data, withUnsafeMutableBytes refers to Data.withUnsafeMutableBytes(_:).

To disambiguate this and refer to the global function, explicitly prefix the module name: _ = Swift.withUnsafeMutableBytes(of: &number) {

Answer 2

Just to complete the set, here's the generic function I created:

/* ###################################################################################################################################### */
// MARK: - Data Extension -
/* ###################################################################################################################################### */
/**
 This extension adds the ability to extract data fron a Data instance, cast into various types.
 */
public extension Data {
    /* ################################################################## */
    /**
     This method allows a Data instance to be cast into various standard types.

     - parameter inValue: This is an inout parameter, and the type will be used to determine the cast.
     - returns: the cast value (the parameter will also be set to the cast value). Can be ignored.
     */
    @discardableResult
    mutating func castInto<T>(_ inValue: inout T) -> T {
        // Makes sure that we don't try to read past the end of the data.
        let len = Swift.min(MemoryLayout<T>.size, self.count)
        _ = Swift.withUnsafeMutableBytes(of: &inValue) {
            self.copyBytes(to: $0, from: 0..<len)
        }

        return inValue
    }
}