protocol CVarArg A type whose instances can be encoded, and appropriately passed, as elements of a C va_list. You use this protocol to present a native Swift interface to a C "varargs" API. For example, a program can import a C API like the one defined here: int c_api(int, va_list arguments) To create a wrapper for the c_api function, write a function that takes CVarArg arguments, and then call the imported C function using the withVaList(_:_:) function: func swiftAPI(_ x: Int, arguments: CVarArg...) -> Int { return withVaList(arguments) { c_api(x, $0) } } Swift only imports C variadic functions that use a va_list for their arguments. C functions that use the ... syntax for variadic arguments are not imported, and therefore can't be called using CVarArg arguments. If you need to pass an optional pointer as a CVarArg argument, use the Int(bitPattern:) initializer to interpret the optional pointer as an Int value, which has the same C variadic calling conventions as a pointer on all supported platforms. Note: Declaring conformance to the CVarArg protocol for types defined outside the standard library is not supported.