how to call a function that takes a function pointer argument in Swift?

Question

According to Apple Documentation:

When calling a function that takes a function pointer argument, you can pass a top-level Swift function, a closure literal, or nil.

I've tried this example from Apple Documentation: Apple Developer

func customCopyDescription(_ p: UnsafeRawPointer?) -> Unmanaged<CFString>? {
    // return an Unmanaged<CFString>? value
}

var callbacks = CFArrayCallBacks(
    version: 0,
    retain: nil,
    release: nil,
    copyDescription: customCopyDescription,
    equal: { (p1, p2) -> DarwinBoolean in
        // return Bool value
}
)

but I got an error message in Xcode: (copyDescription: customCopyDescription error)

A C function pointer can only be formed from a reference to a 'func' or a literal closure

As mentioned at apple documentation , customCopyDescription can be passed as a top-level swift function, but it's seems like something is wrong in the documentation.

How to pass the customCopyDescription func to the CFArrayCallBacks as a swift function(not a closure literal)?

Did you put that inside a class definition? customCopyDescription needs to be a global function. – See also How to use instance method as callback for function which takes only func or literal closure. — Martin R
– Martin R, Commented Feb 4, 2018 at 11:07

Rich Tolley · Accepted Answer · 2018-02-04 11:24:32Z

6

customCopyDescription needs to be a free function, not a method. When I copied your code into Xcode I got the error message only when customCopyDescription was inside a class, not otherwise.

Once placeholder return values are added and customCopyDescription is placed at file scope, the code compiles without a problem

edited Feb 4, 2018 at 11:24

answered Feb 4, 2018 at 11:07

Rich Tolley

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