Tag Archives: memory management

Why I Don’t Care About Swift

Swift is a new programming language created by Apple for use on OS X and iOS devices. The programming world is agog. Apple’s fantastic new language apparently solves all their problems, as evidenced, they say, by the fact that some programmer ported Flappy Birds to it in a few hours.

I’ve been around long enough to see languages come and go. Each claimed to solve all the problems introduced by its predecessors, yet each was replaced by a language that solved all its problems. In some cases, the new language surpassed the success of the language it replaced (C++ and Java); in other cases, the new language faded into obscurity (Modula-2 and Ada).

Lately the motivations for new languages have been dubious. There is a big emphasis on making a language easy to learn and having it hide nasty issues related to memory management and type safety. One review of Swift I read stated, “Apple hopes to make the language more approachable, and hence encourage a new group of self-taught programmers”. While that sounds great, it means that those of us who have mastered our craft after 30 years or more of practice are saddled with the training wheels and water wings that are written into these languages for the noobs.

A classic example is the lack of unsigned integers in Java. The motivation for this was to simplify the language for “new and self-taught programmers” by avoiding errors caused by a lack of understanding of sign-extension. However, for those of us who showed up for class the day that sign extension was taught (that would be day two), we’re left with a language that unnecessarily limits the range of positive integers and requires us to actually have mastered sign extension in order to understand what is happening when we directly manipulate the bits in our integer variables.

Explicit vs. Implicit Typing

One of the simplifications Swift makes is that it infers the types of variables from the values assigned to them rather than requiring the programmer to explicitly type variables. If this was true “weak typing” like I’m familiar with in VBScript, it would be great (though it would come with its own set of problems). But all Swift does is infer the type of the variable from the first value you assign to it.

This actually introduces problems, because it’s not always possible to unequivocally determine the type of a literal value. So Swift gives you ways to force it to interpret a literal value as a given type. Rather than removing the necessity of the programmer understanding types, Swift thus requires “new and self-taught” programmers to have a mastery of types so that they can understand how Swift is working behind the scenes and make sure that their variables have the desired type.

Strings

Swift is said to improve string-handling over Objective-C (the current language used on OS X and iOS). There is certainly room for improvement there. When I first started programming in Objective-C, one of the first things I did was bring over my own C++ string class, as I found NSString to be overly complicated and muddled. Over the years I’ve gotten better with NSString.

I would argue, however, that some of the so-called “improvements” in Swift with respect to strings are differences without a distinction. So instead of this in Objective-C:

[NSString stringWithFormat:@"The value of num is %d", num]

you say this in Swift:

"The value of num is \(num)"

The Swift version is obviously more concise, but it is also less powerful. To add more complex format specifications to Swift you actually have to invoke the functionality of the underlying NSString class, which means the “new and self-taught” programmer, again, needs to understand the details of the implementation in order to do anything beyond the simplest strings.

One of the stated benefits of string handling in Swift is that “all strings are mutable”. One need not worry about whether the string is declared as an NSString (immutable) or NSMutableString (mutable). Well, you don’t have to worry unless you do have to worry — strings assigned to constants are immutable in Swift. So:

var myString1 = "Mutable string"
let myString2 = "Immutable string"
myString1 += myString2    // perfectly legal
myString2 += myString1    // compile-time error

Switch Statements

Swift eliminates the “fall-through” behavior of switch statements, which is said to eliminate bugs caused by omitting the break at the end of each case block. But, oops, sometimes the fall-through behavior is exactly what you want. So Swift adds the fallthrough keyword. It could be argued that Swift eliminates a line of code (the break) while giving the behavior one normally desires. But at the same time, it adds a keyword (fallthrough) that does the opposite. This requires “new and self-taught” programmers to have the same thorough understanding of switch behavior that Objective-C and C++ programmers do.

Single-line Blocks

The Swift compiler will warn you if you omit the braces in any block (such as after an if) and does not allow single-line blocks, thus avoiding this error:

if (x < 0)
    goto fail;
    goto fail;

The code above will always execute one or the other of the goto statements in Objective-C or C++. Even though the second goto is indented, it is not part of the if-block and will be executed if the condition is false.

Swift will warn you about the missing braces and force you to write this:

if (x < 0)
    {
    goto fail;
    }
goto fail;

Or, for those of you who don’t do your braces the right way, this…

if (x < 0) {
   goto fail;
}
goto fail;

This is fine, and hard to argue with. The supposition is that the programmer will immediately recognize the flaw or won’t make the mistake in the first place. On the other hand, I would argue that the same C++ programmer who wrote the erroneous code will write this in Swift:

if (x < 0) {
    goto fail; }
    goto fail;

I always put braces around my blocks, even if they are one-line, so this doesn’t affect me. It’s ironic, however, that while Swift prides itself in eliminating the unnecessary break statement at the end of a case block, it requires two to four additional lines (braces) in if, for, and while statements, which are more numerous.

PocketBible and Swift

I will be more than happy to learn and use Swift for programming on iOS and OS X. I just don’t believe the hype and won’t convert just for the sake of doing something new.

I am a strong proponent of platform-independent languages like C, C++, Java, and, to a lesser extent, Objective-C (the latter is primarily an Apple language, though it has its origins outside of Apple). Such languages allow me to develop code on one platform and re-use it on another. One of the promises of C++ and Java was that you could develop the code for one platform and use it on many others. Swift is an Apple language (the same way C# is a Microsoft language). It only works on Apple devices. While those are numerous, they’re not the only devices out there. So rather than moving toward the “write once, read everywhere” model promised by Java, we’re back to “write everywhere” as each platform requires its own language.

I don’t mind learning a new language. I already jump from C++ to C# to Java to VBScript to Javascript to MS-SQL on a daily basis. For those of us who write code for a living, being multilingual is a job requirement. This is precisely why I care so very little about the supposed advantages of Swift; this isn’t a religious war for me, it’s just a tool. When someone comes out with a new kind of screwdriver, I may or may not buy it until I need it. And then I’ll just buy one and use it — I won’t try to convert all my screwdriver-toting friends.

So will PocketBible for OS X and/or iOS be re-written in Swift? Probably not today, and probably not until Apple requires it. But Swift depends on Objective-C under the hood, so my guess is that Apple will continue to support Objective-C apps for a long time.

Objective-C Memory Management

Perhaps I’m showing my age, but I’m getting awful tired of language designers trying to improve on C/C++ memory management.

Just for review, here’s how memory management should work:

void foo()
  {
  // x is created on the stack. It is deallocated at the end of
  // the block/function and therefore its lifetime matches its
  // scope with no further effort. 

  int x;

  // pX is a pointer to an int that the programmer creates with
  // new. By using "new", the programmer is taking responsibility
  // for freeing the memory used by pX before it goes out of scope.

  int *pX = new int(0);

  // ... interesting code goes here ...

  // The obligatory delete before we exit the block/function.

  delete pX;

  }

Everything else in C/C++ is a variation on this. You can put pointers and variables in structures and classes/objects but they follow the same rules: If you allocate with new, you must free with delete before you lose track of the memory (i.e. the one and only (or last remaining) pointer goes out of scope).

When we started coding for iOS, we ran into “manual retain/release” which is a variation on the C/C++ technique (or rather, a manual method of the automatic garbage collection used in Mac OS):

@interface bar
  {
  // Like C++, when the pointer is a member (instance) variable, 
  // someone else is responsible for allocating memory for it.

  NSString * memberString;
  NSString * anotherString;
  }

// But if the instance variable is accessible from the outside
// world we can say it's a "property" and some of this is 
// managed for us. 

@property (retain) NSString * memberString;

// Unless we don't specify "retain". Now we're responsible for
// making sure the memory for anotherString is allocated and
// freed.

@property (assign) NSString * anotherString;

@end

@implementation bar

- (void)foo
  {
  // These are the same. They're on the stack and are automatically 
  // released when you exit the method/block.

  int x;

  // This is the equivalent of C/C++ "new", kind of. We can't just
  // do memory allocation without also initializing the object 
  // (handled by new and the constructor in C++, but that's the 
  // subject of a different article). The result is a pointer that
  // we're obligated to release before string goes out of scope.

  NSString * string = [[NSString alloc] init];

  // Another way of doing the same thing, but this time the 
  // resulting pointer is automatically released sometime in
  // the future that we don't care about.

  NSString * arString = [[[NSString alloc] init] autorelease];

  // Yet another way of doing the same thing, but the autorelease
  // is done for us. We can tell because the method name starts
  // with something that looks like the name of the class but
  // without the prefix. Intuitively obvious, right?

  NSString * autoString = [[NSString alloc] stringWithUTF8String:"Automatically released"];

  // Required release

  [string release];
  }

@end

And autorelease isn’t as automatic as you might think. You need to think about whether or not you need to create your own autorelease pool. This is important if you’re going to create a large number of autoreleased variables before returning to the run loop. You may want to manage your own autorelease pool in that case so you can free memory up at more convenient times.

If that’s not ridiculous enough, along comes Automatic Reference Counting (ARC) to “simplify” memory management.

@interface bar
  {
  // Like C++, when the pointer is a member (instance) variable, 
  // someone else is responsible for allocating memory for it.

  NSString * memberString;
  NSString * anotherString;
  }

// Instead of "retain", we create a "strong" reference. Memory
// is freed when this particular instance variable goes out
// of scope (is no longer accessible). 

@property (strong) NSString * memberString;

// We use "weak" instead of "assign" to mean that we understand
// someone else is in control of when this memory gets freed.

@property (weak) NSString * anotherString;

@end

@implementation bar

- (void)foo
  {
  // These are the same. They're on the stack and are automatically 
  // released when you exit the method/block. In reality, they're
  // qualified with __strong by default.

  int x = 10;
  NSString * string = [[NSString alloc] init]; // could add __strong for clarity

  // You can also create weak pointers for some reason:

  NSString * __weak weakString;

  // Unfortunately, that introduces a bug into these lines of code:

  weakString = [[NSString alloc] initWithFormat:@"x = %d", x];
  NSLog(@"weakString is '%@'", weakString);

  // In the code above, weakString is immediately deallocated after
  // it is created because there is no strong reference to it. See
  // how this is getting easier?

  // Not to mention:

  NSString * __autoreleasing arString;
  NSString * __unsafe_unretained uuString;

  // Now we don't have to do this:
  // [string release];
  // And that's really all we saved by introducing "Automatic Reference Counting".
  // At the same time, we created a new way to introduce a bug by failing
  // to have any strong pointers to an object from one line of code to
  // the next.
  }

@end

So we’ve gone from:

new / delete

to

retain / release (or autorelease with no release)

to

strong/__strong/weak/__weak/__autoreleasing/__unsafe_unretained

all in the interest of “simplification” and avoiding having to delete/release the memory we allocate. I frankly don’t see the benefits.