How to print an Objective-C pointer with os_trace?

Question

I'm trying to se Activity Tracing in an iOS app, things have been working nicely up until I needed to print an Objective-C pointer.

In the slides for WWDC 14 Session 714 Apple mention that you can use %p to print pointers:

But I can't find a reliable way to do so without up setting ARC (or myself).

I want to print the NSManagedObjectContext associated with a NSManagedObject, because I use multiple context I want to be able to see if I'm passing NSManagedObject to unexpected contexts.

1st try: ARC forbids Objective-C objects in structs

os_trace("doing something with object in moc %p", object.managedObjectContext);

2nd try: Compiles but don't print what I want

NSManagedObjectContext *moc = invite.managedObjectContext;
os_trace("doing something with object in moc %p", &moc);

The resulting trace prints 0x16fd0e6d0 while (lldb) po invite.managedObjectContext says that the pointer is 0x1741fce00.

3rd try: I can't believe this is the only way

CFTypeRef moc = CFBridgingRetain(object.managedObjectContext);
os_trace("doing something with object in moc %p", moc);
CFRelease(moc);

This prints the address I'm expecting. But OMG having to manually retain/release something just to print the memory address is horrible.

What am I missing? There has to be a better way to do this.

Where you say "this isn't what I want" you define `NSManagedObjectContext *moc` perhaps you should pass `moc` as the argument to the format spec`%p`, not `&moc`. — Weather Vane, Sep 04 '15 at 23:04
@WeatherVane If I do that I get the same error O get on the 1st try version. — Felipe Cypriano, Sep 04 '15 at 23:06
@WeatherVane you're right, with a little more because we need a bridge cast in ARC, so `(__bridge void *)moc` — Felipe Cypriano, Sep 04 '15 at 23:19
@WeatherVane Feel free to add that as an answer and I'll mark it as correct. BTW the void cast also works with the 1st try code. — Felipe Cypriano, Sep 04 '15 at 23:52

score 0 · Accepted Answer · answered Sep 05 '15 at 18:03

0

The correct way of passing a pointer to the %p argument of a print formatter, is by casting it as (void*).

answered Sep 05 '15 at 18:03

Weather Vane

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How to print an Objective-C pointer with os_trace?

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