[swift-users] Convenience initializers in structs?

Jens Persson jens at bitcycle.com
Tue Jul 18 16:22:23 CDT 2017

That is not true. Structs can have delegating initializers but they cannot
be marked with `convenience` (only the initializers of class types can).

This is very clear from both the documentation and the compiler:

(1) The Swift Programming language (Swift 4):
"Swift defines two kinds of initializers for *class* types to help ensure
all stored properties receive an initial value. These are known as
designated initializers and convenience initializers."

(2) The compiler:

struct S {
    var a, b: Int
    init(_ a: Int, _ b: Int) {
        self.a = a
        self.b = b
    convenience init(_ ab: Int) { // <-- Error
        self.init(ab, ab)
The error message is:
Delegating initializers in structs are not marked with 'convenience'
The suggested fix is to remove the word convenience.

Please reread my previous post, perform the steps I describe (looking at
Quick Help, also try jump to definition for that `init` and you'll see it
is marked with `convenience` even though it is in a struct), also look at
the link to the documentation for the Int init, it too is marked with
convenience, even though Int is a struct.


On Tue, Jul 18, 2017 at 9:46 PM, Slava Pestov <spestov at apple.com> wrote:

> Hi Jens,
> While I’m not familiar with the integer API in the standard library,
> structs and enums certainly can have convenience initializers. They must
> delegate to another initializer (either convenience or designated) rather
> than initializing the fields of the type one by one.
> Slava
> On Jul 18, 2017, at 6:46 AM, Jens Persson via swift-users <
> swift-users at swift.org> wrote:
> Start a command line project in Xcode 9 beta 3 and copy paste this single
> line of code into main.swift
> let _ = UInt8.init(extendingOrTruncating: UInt64(123456))
> Now look at Quick Help while placing the cursor on `init` and then on
> `extendingOrTruncating`.
> Note that (and how) the documentation for the initializer differs
> depending on where you place the cursor.
> If the cursor is on `init`, the initializer is shown to be a
> convenience(!) initializer even though structs (such as UInt8) cannot have
> convenience initializers, right?
> Even the official documentation for this and several other initializer
> like eg:
> https://developer.apple.com/documentation/swift/int/2885075-init
> clearly shows convenience initializers in structs.
> By the way, .init(extendingOrTruncating:) doesn't show in the list of
> completions for "UInt8.init" but it does for "UInt8(".
> Can anyone explain what's going on?
> Are these known issues that will go away in time for Xcode 9 GM?
> /Jens
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