Passing struct from ObjC to Swift

Question

Trying to use a struct from ObjC to Swift doesn't seem to be that easy. I end up getting a Unsafe pointer that I don't know if I can cast reliably.

Here is the code:

//
// In OBJC land
//

// Type declared as a struct

typedef struct node {
    int children_count;
} node_t;

// Super class has a property

@property (nonatomic, assign, readonly) node_t *node;


//
// In SWIFT land
//

// Derived class tries to set the property inside the C struct


let n: UnsafeMutablePointer<node_t> = super.node // As swift compiler sees it
n.children_count = 0    // ERR!!!

Do I really need to apply unsafeBitcast here or is there a simpler/safer and more elegant way to convert what seems to be a frequent scenario?

UPDATE:

I tried using memory to access the elements of the struct and I am getting a EXC_BAD_INSTRUCTION

var node: node_t = self.node.memory
node.children_count = 42

UPDATE CONT'D & FINALE

I got it to work. Thanks to @matt's patience for making sure I groked 'memory' access completely. The other trick is to realize the assignment in one continuous statement like this:

    var node: UnsafeMutablePointer<node_t> = self.node
    node.memory.children_count = 42

If I do the following, the change doesn't get committed passed the function call:

var node: node_t = self.node.memory
node.children_count = 42

"If I do the following, the change doesn't get committed passed the function call" Ah, very good point; yes, because in Swift the assignment of the struct to node causes that struct to be copied. — matt
– matt, Commented Jun 7, 2015 at 21:51
Thanks for sticking with it - I really was prepared to send you a demo project if you needed it! :) — matt
– matt, Commented Jun 7, 2015 at 21:54

matt · Accepted Answer · 2015-06-07 18:28:13Z

1

One problem is that this is not a "frequent scenario". In fact, this is a very odd thing to do:

@property (nonatomic, assign, readonly) node_t *node;

This thing (node_t) is a C struct. So why do you want a pointer to it? Perhaps you have no choice about this, but far and away the usual thing is simply to have the struct itself as a property:

@property (readonly) node_t *node;

In that way, the property arrives as a Swift struct called node_t, and if you have a var reference to it (not a let as you've written it), you can assign into its children_count directly, as you would expect.

If you insist on having a pointer, then you will have to dereference the pointer, won't you? You are not doing that. To do so, take the pointer's memory:

n.memory.children_count = 0

edited Jun 7, 2015 at 18:28

answered Jun 7, 2015 at 18:24

matt

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8 Comments

John Difool Over a year ago

Thanks for your answer. However, as you guessed, I don't have any choice but to use what's in there. The code is inherited from a objectiveC framework that I can't modify. I need a way to bridge the struct somehow. I hope there is a way.

matt Over a year ago

See my revised answer. - Yes, I did guess that. :)

matt Over a year ago

Also, are you sure you need super? If we are the subclass, we inherit this instance variable and self should work.

John Difool Over a year ago

I merely used 'super' to examplify my use case (just to show that I am indeed inheriting it.) Terrible code that I wouldn't check in. Thanks for code reviewing with an eagle eye though.

matt Over a year ago

So how about that little memory trick - is that working? (Of course it assumes that this property has been correctly initialized - i.e. the pointer is in fact pointing at something - but that is not a part of the question. If the pointer is not pointing at the desired struct, you would obviously crash.)

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