ios - swift 错误 : Cannot pass immutable value as inout argument: 'pChData' is a 'let' constant

Question

我有一个如下所示的函数:

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)
}

获取错误:

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

虽然我在这里传递的参数都不是 let 常量。为什么我会得到这个？

Answer 1

默认情况下，传递给函数的参数在函数内部是不可变的。

您需要制作一个变量副本(兼容 Swift 3):

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)
}

或者，对于 Swift 2，您可以将 var 添加到参数中:

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)
}

第三个选项，但这不是您要的:使参数成为 inout。但它也会在函数的 外部 改变 pchData，所以看起来你不想在这里使用它 - 这不在你的问题中(但我当然可能读错了)。