Swift Pointer Arithmetic for C Interoperability

PointerArithmetic.swift

struct MyStruct {
    var n: Int
    
    func sayHello() {
        print("hello")
    }
}

var s: MyStruct = MyStruct(n: 1)
var a: [UInt8] = []

func takeVoidPtr(_ p: UnsafeMutableRawPointer){
    
}

func takeTypedMyStructPtr(_ p: UnsafeMutablePointer<MyStruct>){
    
}

func takeTypedUInt8Ptr(_ p: UnsafeMutablePointer<UInt8>){
    
}

// void pointers
do {
    let sp = UnsafeMutableRawPointer(&s)
    let ap = UnsafeMutableRawPointer(&a)
    
    // cast to typed
    let ssp = sp.assumingMemoryBound(to: MyStruct.self)
    let aap = ap.assumingMemoryBound(to: Array<UInt8>.self)
}

// typed pointers
do {
    let sp = UnsafeMutablePointer(&s)
    let ap = UnsafeMutablePointer(&a)
    
    // cast to void
    let ssp = UnsafeMutableRawPointer(sp)
    let aap = UnsafeMutableRawPointer(ap)
    
    // reconstruct object from void pointer
    let ss = ssp.assumingMemoryBound(to: MyStruct.self).pointee
    let aa = ssp.assumingMemoryBound(to: Array<UInt8>.self).pointee
}

// struct pointers safely
do {
    withUnsafeMutablePointer(to: &s) { p in
        takeVoidPtr(p)
    }
    
    withUnsafeMutablePointer(to: &s) { p in
        takeTypedMyStructPtr(p)
    }
}

// using array pointers safely
do {
    a.withUnsafeMutableBytes { p in
        takeVoidPtr(p.baseAddress!)
    }
    a.withUnsafeMutableBufferPointer { p in
        takeTypedUInt8Ptr(p.baseAddress!)
    }
    // the following would generate a UnsafeMutablePointer<[UInt8]> and does not compile
    // automatic converstion does not work for withUnsafeMutablePointer
    // see also https://bugs.swift.org/browse/SR-12442
    // withUnsafeMutablePointer(to: &s) { p in
    //    takeTypedUInt8Ptr(p) // Cannot convert value of type 'UnsafeMutablePointer<MyStruct>' to expected argument type 'UnsafeMutablePointer<UInt8>'
    // }
}