UnsafeMutablePointer calling function from Swift

Question

I have a method defined as below in Objective-C:

- (BOOL)dataHandler:(void*)buffer length:(UInt32)length

In ObjC we call this method as so:

self [self dataHandler:(void*)[myNsString UTF8String] length:[myNsString length]];

When I see the prototype for this for Swift it comes out like this:

self.dataHandler(<#buffer: UnsafeMutablePointer<Void>#>, length: <#UInt32#>)

I'm trying it as per below and not able to make it work:

self.dataHandler(buffer: UnsafeMutablePointer(response!), length: count(response!))

I also tried:

 var valueToSend = UnsafeMutablePointer<Void>(Unmanaged<NSString>.passRetained(response!).toOpaque())
 self.dataHandler(buffer: valueToSend, length: count(response!))

Is there anything else I haven't tried? I read the below on this:

Get the length of a String

UnsafeMutablePointer<Int8> from String in Swift

What type has response?

Martin R
– Martin R

2015-09-16 19:10:14 +00:00
Commented Sep 16, 2015 at 19:10 — Martin R
– Martin R, Commented Sep 16, 2015 at 19:10
It was a String in Swift

KVISH
– KVISH

2015-09-16 19:10:43 +00:00
Commented Sep 16, 2015 at 19:10 — KVISH
– KVISH, Commented Sep 16, 2015 at 19:10

Community · Accepted Answer · 2017-05-23 12:24:35Z

3

Similar as in UnsafeMutablePointer<Int8> from String in Swift, you can send the response string as UTF-8 encoded bytes to the handler method with

response.withCString {
    self.dataHandler(UnsafeMutablePointer($0), length: UInt32(strlen($0)))
}

Inside the block, $0 is a pointer to a NUL-terminated array of char with the UTF-8 representation of the string.

Note also that your Objective-C code

[self dataHandler:(void*)[myNsString UTF8String] length:[myNsString length]]

has a potential problem: length returns the number of UTF-16 code points in the string, not the number of UTF-8 bytes. So this will not pass the correct number of bytes for strings with non-ASCII characters.

edited May 23, 2017 at 12:24

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answered Sep 16, 2015 at 19:21

Martin R

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2 Comments

KVISH Over a year ago

Can you explain to me what it does? And why does it have to be response.withCString { }, why can't it directly calling dataHandler?

Martin R Over a year ago

@KVISH: See update. You cannot call it directly because String is not a C string but a struct containing pointers to a representation of the string.

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