Representing NULL Function Pointers to C Functions in Swift

Question

Consider the private-yet-sort-of-documented Cocoa C functions _NSLogCStringFunction() and _NSSetLogCStringFunction(). _NSLogCStringFunction(

Answer 1

The Swift mapping of the (Objective-)C declaration

extern void _NSSetLogCStringFunction(void(*)(const char*, unsigned, BOOL));

is

public func _NSSetLogCStringFunction(_: (@convention(c) (UnsafePointer, UInt32, ObjCBool) -> Void)!)

The easiest solution would be to put the Objective-C extern declaration into an Objective-C header file and include that from the bridging header.

Alternatively, in pure Swift it should be

typealias NSLogCStringFunc = @convention(c) (UnsafePointer, UInt32, ObjCBool) -> Void

@_silgen_name("_NSSetLogCStringFunction")
func _NSSetLogCStringFunction(_: NSLogCStringFunc!) -> Void

In either case, the function parameter is an implicitly unwrapped optional, and you can call it with nil. Example:

func myLogger(message: UnsafePointer, _ length: UInt32, _ withSysLogBanner: ObjCBool) -> Void {
    print(String(format:"myLogger: %s", message))
}

_NSSetLogCStringFunction(myLogger) // Set NSLog hook.
NSLog("foo")
_NSSetLogCStringFunction(nil) // Reset to default.
NSLog("bar")

Output:

myLogger: foo
2016-04-28 18:24:05.492 prog[29953:444704] bar