Swift Error: Cannot pass immutable value as inout argument: 'pChData' is a 'let' constant

Question

I have a function that looks like the following:

func receivedData(pChData: UInt8, andLength len: CInt) {
    var receivedData: Byte = Byte()
    var receivedDataLength: CInt = 0

    memcpy(&receivedData, &pChData, Int(len));  // Getting the error here
    receivedDataLength = len
    AudioHandler.sharedInstance.receiverAudio(&receivedData, WithLen: receivedDataLength)
}

Getting the error:

Cannot pass immutable value as inout argument: 'pChData' is a 'let' constant

Xcode screenshot

Though none of the arguments which I am passing here are let constants. Why am I getting this?

Does your function need to be able to mutate pChData inside *and* outside its scope? Or only inside? It isn't clear from your question. Be careful to not use inout if you only need the value to be a variable *inside*. — Eric Aya, May 03 '16 at 17:37
@HarshalBhavsar `memcpy()´ is a function defined in `Darwin.C.string`. One of its uses is modifying an MTLBuffer. — Andreas is moving to Codidact, Sep 14 '17 at 15:28

score 8 · Answer 1 · answered May 03 '16 at 17:15

Arguments passed to a function are immutable inside the function by default.

You need to make a variable copy (Swift 3 compatible):

func receivedData(pChData: UInt8, andLength len: CInt) {
    var pChData = pChData
    var receivedData: Byte = Byte()
    var receivedDataLength: CInt = 0

    memcpy(&receivedData, &pChData, Int(len));  // Getting the error here
    receivedDataLength = len
    AudioHandler.sharedInstance.receiverAudio(&receivedData, WithLen: receivedDataLength)
}

or, with Swift 2, you can add var to the argument:

func receivedData(var pChData: UInt8, andLength len: CInt) {
    var receivedData: Byte = Byte()
    var receivedDataLength: CInt = 0

    memcpy(&receivedData, &pChData, Int(len));  // Getting the error here
    receivedDataLength = len
    AudioHandler.sharedInstance.receiverAudio(&receivedData, WithLen: receivedDataLength)
}

Third option, but that's not what you're asking for: make the argument an inout. But it will also mutate pchData outside of the func, so it looks like you don't want this here - this is not in your question (but I could have read that wrong of course).

score 6 · Accepted Answer · answered May 03 '16 at 17:05

You are trying to access/modify pChData argument, which you can't unless or until you declare it as inout parameter. Learn more about inout parameter here. So try with the following code.

func receivedData(inout pChData: UInt8, andLength len: CInt) {
    var receivedData: Byte = Byte()
    var receivedDataLength: CInt = 0

    memcpy(&receivedData, &pChData, Int(len));
    receivedDataLength = len
    AudioHandler.sharedInstance.receiverAudio(&receivedData, WithLen: receivedDataLength)
}

In Swift4，pChData should declared as 'pChData: inout UInt8'. — ooOlly, Nov 19 '18 at 13:49

Swift Error: Cannot pass immutable value as inout argument: 'pChData' is a 'let' constant

2 Answers2