如何从[UnsafeMutablePointer投射元素 <UInt8> 在Swift中转为UInt8 *在C ++中

Question

I have the following code that will not compile because XCode won't let me cast a NSArray element into a pointer in my C++ code. The error given by XCode is: Assigning to 'UInt8 *' from incompatible type 'id' .

How am I supposed to pass an array of type [UnsafeMutablePointer<UInt8>] from Swift to Objective-C++ ?

Thank you in advance

objcfunc.h

+ (void) call: (NSArray *) arr;

objcfunc.mm

+ (void) call: (NSArray *) arr {
 UInt8 *buffer;
 buffer = (UInt8 *) arr[0]; // doesn't work, XCode throws an error
 unsigned char *image;
 image = (unsigned char *) buffer;
 processImage(image); // C++ function
}

Swift

var arr: [UnsafeMutablePointer<UInt8>] = []
arr.append(someImage)
objcfunc.call(swiftArray: arr)

But if I don't use an array and directly pass the pointer, the code works fine:

objcfunc.h

+ (void) callSingle: (UInt8 *) buf;

objcfunc.mm

+(void) callSingle: (UInt8 *) buf {
unsigned char *image;
image = (unsigned char *) buf; // works fine
processImage(image);
}

Swift

let x: UnsafeMutablePointer<UInt8> buf;
// initialize buf
objcfunc.callSingle(buf);

Answer 1

NSArray is an array of Objective-C objects. So, you need to pass an array of instances of types that are bridged to Objective-C types. I'm not sure that Swift's UnsafeMutablePointer struct is bridged.

Because in this case you are passing an array of image buffers (if I correctly understand), you might want to consider using NSData or Data , rather than UnsafeMutablePointer<UInt8> for each image buffer. These types are specifically intended for dealing with an array of bytes, which is what an image buffer is; see https://developer.apple.com/reference/foundation/nsdata and https://developer.apple.com/reference/foundation/data

Here is a contrived example of how this could be done using Data and NSData :

Objective-C implementation:

@implementation MyObjC

+ (void) call: (NSArray * ) arr {
    NSData * data1 = arr[0];
    UInt8 * bytes1 = (UInt8 *)data1.bytes;
    bytes1[0] = 222;
}

@end

Swift:

var arr: [UnsafeMutablePointer<UInt8>] = []

// This is just an example; I'm sure that actual initialization of someImage is more sophisticated.
var someImage = UnsafeMutablePointer<UInt8>.allocate(capacity: 3)
someImage[0] = 1
someImage[1] = 12
someImage[2] = 123

// Create a Data instance; we need to know the size of the image buffer.
var data = Data(bytesNoCopy: someImage, count: 3, deallocator: .none)

var arrData = [data]  // For demonstration purposes, this is just a single element array
MyObjC.call(arrData)  // You may need to also pass an array of image buffer sizes.

print("After the call: \(someImage[0])")