linker command failed

With the advent of Swift in the Apple Developer ecosystem, I’ve been learning quite a bit about the way that things work under the hood. Let’s get one thing straight, though: I am not a trained computer programmer. I’ve tried to take algorithm classes, but everything goes over my head because I don’t really want to spend the time figuring out what everything actually does. Everything I have learned is from trial and error (mostly through the errors).

Apple Bug Reporting has previously been referred to as Radar, which is the name of the bug tracking tool. Recently, I've been sending bug reports for the new version of Swift. Version 1.2 had this issue that crashed the compiler with a SegFault:

Often times, I find myself looking at something in Swift and wondering, "What class is this object?" which is something with an easy answer in Objective-C:

[someObject class]

But in Swift, a different question becomes a lot more important: "Is this a certain type of class?"

let anObject = someObject as? SomeClass

Note that this assignment creates anObject, a variable of type SomeClass?, which is an Optional -- specifically, a type of object that can potentially be nil. Swift is specific about non-optional types returning a value always, so Optionals must be used in situations where a nil value might be returned:

override func tableView(tableView: UITableView, willSelectRowAtIndexPath indexPath: NSIndexPath) -> NSIndexPath?

Notice that Apple expects this function to return an Optional NSIndexPath. According to Apple, this function "Tells the delegate that the specified row is now selected." What happens if we want to do something, but not select the row? Simply return nil.

Since optional assignment can return nil, the assignment itself can be used as the argument of an if statement (something that I remember being excited about when learning PHP in high school). But this leads to a really easy use that I came up with today when implementing a custom isEqual method on a class I have been writing. It's completely obvious if you are looking at things from the Swift perspective, but something that really just clicked for me today.

Here's a way I might check for equality of a custom object in Objective-C:

-(BOOL) isEqual:(id)object {     if ( ![object isKindOfClass:[self class]] ) {             //test important, identifying features             if( object.property1 == self.property1 &&         object.property2 == self.property2 ... etc) {                return YES;          }      }      return NO;  }

Here's what I've used in Swift, using the ubiquitous Optional assignment:

override func isEqual(object: AnyObject) {     if let someObject = object as? SomeClass {         //test important, identifying features here         if someObject.property1 == someObject.property2 etc {             return true         }     }     return false }

I ran across my favorite error again today. Often times these are really difficult to understand if you don't know what they are referring to. Here's the one that I got:

Undefined symbols for architecture x86_64:

  "_CATransform3DMakeRotation", referenced from:

      -[SOFViewController updateScaleName] in SOFViewController.o

ld: symbol(s) not found for architecture x86_64

clang: error: linker command failed with exit code 1 (use -v to see invocation)

Turns out this is super easy. In this case, the symbol is undefined because the QuartzCore library isn't included, despite the inclusion of #include <QuartzCore/QuartzCore.h>.

Hey, shortcuts are nice, but what about this Objective-C monstrosity:

NSMutableArray *returnArray = [@[] mutableCopy];

Guh. I kind of wrote this because I could, but I think this is much better:

NSMutableArray *returnArray = [NSMutableArray new];

While we're at it, since this is for a function that is going to return an immutable NSArray, why not go for a name that reflects what is actually going on?

NSMutableArray *mutableReturnArray = [NSMutableArray new];

Finally, when it's time:

return [mutableReturnArray copy];

I've been using git for about a year now. Today, I refactored some code and hit a personal milestone:

9 files changed, 153 insertions(+), 162 deletions(-)

First time that a refactor has removed more code than it's inserted. I feel pretty good about that, but it was pretty easy, seeing as there was a bunch of redundant, encapsulated stuff that had no business spanning multiple methods.

Linker Command turns one year old today, and I'm in my last days at the place that I started it for the Summer, so I wanted to share the error behind the name.

Last summer, I was getting really angry at seeing this error come up over and over again. If you're new to C like I am, and you something that starts like this:

Ld /Volumes/Storage/Users/mike/Library/Developer/Xcode/DerivedData/PedalCharts-gvryltpjsvqnwraultwngecnzbsd/Build/Products/Debug-iphonesimulator/PedalCharts.app/PedalCharts normal x86_64

    cd /Volumes/Storage/Users/mike/Documents/Cocoa/Bitbucket/PedalCharts

    export IPHONEOS_DEPLOYMENT_TARGET=7.0

    export PATH="/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin:/Applications/Xcode.app/Contents/Developer/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin"

    ETC ETC ETC

You are going to look at this and say, I have no idea what to do with this garbage. The important stuff is in the second half (and Xcode is much more forthcoming with this than it used to be):

duplicate symbol _OBJC_METACLASS_$_PedalChart in:

    /Volumes/Storage/Users/mike/Library/Developer/Xcode/DerivedData/PedalCharts-gvryltpjsvqnwraultwngecnzbsd/Build/Intermediates/PedalCharts.build/Debug-iphonesimulator/PedalCharts.build/Objects-normal/x86_64/PCStaffView.o

    /Volumes/Storage/Users/mike/Library/Developer/Xcode/DerivedData/PedalCharts-gvryltpjsvqnwraultwngecnzbsd/Build/Intermediates/PedalCharts.build/Debug-iphonesimulator/PedalCharts.build/Objects-normal/x86_64/PedalChart-45484887584B8F82.o

duplicate symbol _OBJC_CLASS_$_PedalChart in:

    /Volumes/Storage/Users/mike/Library/Developer/Xcode/DerivedData/PedalCharts-gvryltpjsvqnwraultwngecnzbsd/Build/Intermediates/PedalCharts.build/Debug-iphonesimulator/PedalCharts.build/Objects-normal/x86_64/PCStaffView.o

    /Volumes/Storage/Users/mike/Library/Developer/Xcode/DerivedData/PedalCharts-gvryltpjsvqnwraultwngecnzbsd/Build/Intermediates/PedalCharts.build/Debug-iphonesimulator/PedalCharts.build/Objects-normal/x86_64/PedalChart-45484887584B8F82.o

ld: 2 duplicate symbols for architecture x86_64

Along with, finally, the familiar:

clang: error: linker command failed with exit code 1 (use -v to see invocation)

The important part here is the duplicate symbols. The linker is part of the C architecture that links the functions and variables you write with the places they are used in your code. In this case, the linker is complaining that the class PedalChart has been defined in two places. Today, it's because I used this typo in my PCStaffView Class:

#import "PedalChart.m"

So, this is a big problem, because, instead of importing the header with the declaration of the class, I've tried to import the implementation file. The linker doesn't like this and will throw the error on build time. This is the most likely cause of the error for me, and it's due to a typo that can be autocompleted.

I thought it might be interesting to share the differences between the isEqual methods used by NSDictionary and my new class to shed some more light on the situation.

Here's the way I set up Skill Plan items prior to writing the custom class:

[[NSDictionary dictionaryWithObjectsAndKeys: [EVESkill skillWithID:@2345], @"skill", [NSNumber numberWithInteger:1], @"level", nil];

Here's the way that I set them up now:

[EVESkillPlanItem skillPlanItemWithSkill: [EVESkill skillWithID:@2345] atLevel:1];

I'm embarking on the most difficult project I've written so far, and I have been making extensive use of Instruments, especially the Time Profiler. If you're like me, you opened up Instruments after downloading Xcode for the first time because it looked cool. Then you shut the computer and cried for a while. But recently, I've been noticing that my application tends to be sluggish during some dragging operations, and I wanted to figure out precisely why.

First, let's talk a little about the project and why it slows down where it does. I've been playing a game called EVE Online. One of the core mechanics of this MMO is that your character trains skills 24 hours a day, whether you are online or not. But skills can only be queued if they will start within 24 hours. And certain skills can't even be trained until other skills have been reached. As a result, people really want help in planning longer skill queues. There's a really good program for this called EVEMon... which runs only on the PC.

Enter fledgling developer. EVE Online has not been available on the mac for very long, so the options for us are quite slim. I'm hoping to come up with something that works well and eliminates the need for Wine (which doesn't run EVEMon well) or Bootcamp (which is annoying).

So, I've downloaded all of the skill information, set it up in arrays and dictionaries, etc etc. Next I created a class called EVESkillPlan, which works with my EVESkill items. These are my Models (although I've integrated some Controller functionality in the EVESkillPlan, which is supposed to be a big no-no -- more on this in another post).