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I am tasked with converting some functions from Objective-C to Swift. We are using a C file to transfer files to a bluetooth device. Here is the Objective-C code that needs to be converted.

From the Objective-C file:

NSString *room_name = filename;
NSData* bytes = [room_name dataUsingEncoding:NSUTF8StringEncoding];
UInt8 buff_name[bytes.length+1];
memcpy(buff_name, [room_name UTF8String],[room_name lengthOfBytesUsingEncoding:NSUTF8StringEncoding] + 1);

I am stuck on memcpy for Swift and am not sure this is the safest method for what we are trying to do. Thank you.

My condolences, as the code you were given is a bit of a mess. First, it makes an NSData from room_name, converting the string to UTF-8 in the process, but then it doesn't use that NSData for anything other than to determine the size of a buffer, and then it converts the string to UTF-8 again and copies it into the new buffer. This is a lot more inefficient, both from a computational and a readability standpoint, than it needs to be.

If we were leaving this in Objective-C, we'd already want to clean up the code. However, in Swift, we can make this even cleaner, because String has a really nice and simple method to get us a pointer to the string's UTF-8 representation:

filename.withCString ptr in
 // call whatever C-based functions expect a `const char *` pointer here

Note that the pointer you get here is immutable. If the C functions you're calling expect to be able to modify the buffer, you'll want to make a copy instead. There are a few ways to do this, but the easiest is probably just to:

guard var data = filename.data(using: .utf8) else 
 // handle this error somehow


data.withUnsafeMutableBytes (ptr: UnsafeMutablePointer<Int8>) in
 // call whatever C-based functions expect a `char *` pointer here
 // (if you need `unsigned char *`, use `UInt8` instead of `Int8`)