What are the difference and use-cases for va_list, CVaListPointer, AnyObject …, CVarArgType?

Question

Question

Can someone please explain the differences between these argument types? Furthermore, if possible please supply appropriate use-cases using cod

Answer 1

va_list is a C type used for variable argument functions. You'll see this as a parameter in Objective-C code.

CVaListPointer is the swift equivalent of a va_list--wherever you see a function in Objective-C that takes va_list as an argument, you'll pass in a CVaListPointer in Swift.

objective-c: (NSPredicate *)predicateWithFormat:(NSString *)format arguments:(va_list)argList
swift: init(format format: String, arguments argList: CVaListPointer) -> NSPredicate

CVarArgType is a protocol that defines what kind of types can be included in a CVaListPointer, this includes all the primitives (Int, Float, UInt, etc) as well as COpaquePointer

The utility function withVaList takes a Swift array and transforms it into a CValListPointer, which is then passed to a callback. Note that the array passed in must contain only CVarArgType variables:

var format = "%@ != %@"
var args: [CVarArgType] = ["abc", "def"]

var s = withVaList(args) { (pointer: CVaListPointer) -> NSPredicate in
    return NSPredicate(format: format, arguments: pointer)
}

Swift also defines its own varadic parameter T..., which must be the last parameter in a function, it is passed into the function as [T] and is used like so.

var arg1: String = "abc"
var arg2: String = "def"
NSPredicate(format: format, arg1, arg2)

Use Swift's built in varadic parameters T... wherever possible. CValListPointer should only be used when you need to interface with Objective C and C APIs that accept va_list arguments.