Objective-C 常见陷阱

#Original (English)

Source: https://www.mikeash.com/pyblog/friday-qa-2012-12-14-objective-c-pitfalls.html

Objective-C is a powerful and extremely useful language, but it’s also a bit dangerous. For today’s article, my colleague Chris Denter suggested that I talk about pitfalls in Objective-C and Cocoa, inspired by Cay S. Horstmann’s article on C++ pitfalls.

IntroductionI’ll use the same definition as Horstmann: a pitfall is code that compiles, links, runs, but doesn’t do what you might expect it to. He provides this example, which is just as problematic in Objective-C as it is in C++:

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    if (-0.5 <= x <= 0.5) return 0;

A naive reading of this code would be that it checks to see whether x is in the range [-0.5, 0.5]. However, that’s not the case. Instead, the comparison gets evaluated like this:

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    if ((-0.5 <= x) <= 0.5)

In C, the value of a comparison expression is an int, either 0 or 1, a legacy from when C had no built-in boolean type. It is that 0 or 1, not the value of x, that is compared with 0.5. In effect, the second comparison works as an extremely weirdly phrased negation operator, such that the if statement’s body will execute if and only if x is less than -0.5.

Nil ComparisonObjective-C is highly unusual in that sending messages to nil does nothing and simply returns 0. In nearly every other language you’re likely to encounter, the equivalent is either prohibited by the type system or produces a runtime error. This can be both good and bad. Given the subject of the article, we’ll concentrate on the bad.

First, let’s look at equality testing:

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    [nil isEqual: @"string"]

Messaging nil returns 0, which in this case is equivalent to NO. That happens to be the correct answer here, so we’re off to a good start! However, consider this:

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    [nil isEqual: nil]

This also returns NO. It doesn’t matter that the argument is the exact same value. The argument’s value doesn’t matter at all, because messages to nil always return 0 no matter what. So going by isEqual:, nil never equals anything, including itself. Mostly right, but not always.

Finally, consider one more permutation with nil:

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    [@"string" isEqual: nil]

What does this do? Well, we can’t be sure. It may return NO. It may throw an exception. It may simply crash. Passing nil to a method that doesn’t explicitly say it’s allowed is a bad idea, and isEqual: doesn’t say that it accepts nil.

Many Cocoa classes also include a compare: method. This takes another object of the same class and returns either NSOrderedAscending, NSOrderedSame, or NSOrderedDescending, to indicate less than, equal, or greater than.

What happens if we compare with nil?

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    [nil compare: nil]

This returns 0, which happens to be equal to NSOrderedSame. Unlike isEqual:, compare: thinks nil equals nil. Handy! However:

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    [nil compare: @"string"]

This also returns NSOrderedSame, which is definitely the wrong answer. compare: will consider nil to be equal to anything and everything.

Finally, just like isEqual:, passing nil as the parameter is a bad idea:

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    [@"string" compare: nil]

In short, be careful with nil and comparisons. It really just doesn’t work right. If there’s any chance your code will encounter nil, you must check for and handle it separately before you start doing isEqual: or compare:.

HashingYou write a little class to contain some data. You have multiple equivalent instances of this class, so you implement isEqual: so that those instances will be treated as equal. Then you start adding your objects to an NSSet and things start behaving strangely. The set claims to hold multiple objects after you just added one. It can’t find stuff you just added. It may even crash or corrupt memory.

This can happen if you implement isEqual: but don’t implement hash. A lot of Cocoa code requires that if two objects compare as equal, they will also have the same hash. If you only override isEqual:, you violate that requirement. Any time you override isEqual:, always override hash at the same time. For more information, see my article on Implementing Equality and Hashing.

MacrosImagine you’re writing some unit tests. You have a method that’s supposed to return an array containing a single object, so you write a test to verify that:

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    STAssertEqualObjects([obj method], @[ @"expected" ], @"Didn't get the expected array");

This uses the new literals syntax to keep things short. Nice, right?

Now we have another method that returns two objects, so we write a test for that:

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    STAssertEqualObjects([obj methodTwo], @[ @"expected1", @"expected2" ], @"Didn't get the expected array");

Suddenly, the code fails to compile and produces completely bizarre errors. What’s going on?

What’s going on is that STAssertEqualObjects is a macro. Macros are expanded by the preprocessor, and the preprocessor is an ancient and fairly dumb program that doesn’t know anything about modern Objective-C syntax, or for that matter modern C syntax. The preprocessor splits macro arguments on commas. It’s smart enough to know that parentheses can nest, so this is seen as three arguments:

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    Macro(a, (b, c), d)

Where the first argument is a, the second is (b, c), and the third is d. However, the preprocessor has no idea that it should do the same thing for [] and {}. With the above macro, the preprocessor sees four arguments:

• [obj methodTwo]

• @[ @“expected1”

• @“expected2 ]

• @“Didn’t get the expected array”

This results in completely mangled code that not only doesn’t compile, but confuses the compiler beyond the ability to provide understandable diagnostics. The solution is easy, once you know what the problem is. Just parenthesize the literal so the preprocessor treats it as one argument:

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    STAssertEqualObjects([obj methodTwo], (@[ @"expected1", @"expected2" ]), @"Didn't get the expected array");

Unit tests are where I’ve run into this most frequently, but it can pop up any time there’s a macro. Objective-C literals will fall victim, as will C compound literals. Blocks can also be problematic if you use the comma operator within them, which is rare but legal. You can see that Apple thought about this problem with their Block_copy and Block_release macros in /usr/include/Block.h:

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    #define Block_copy(...) ((__typeof(__VA_ARGS__))_Block_copy((const void *)(__VA_ARGS__)))
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    #define Block_release(...) _Block_release((const void *)(__VA_ARGS__))

These macros conceptually take a single argument, but they’re declared to take variable arguments to avoid this problem. By taking … and using VA_ARGS to refer to “the argument”, multiple “arguments” with commas are reproduced in the macro’s output. You can take the same approach to make your own macros safe from this problem, although it only works on the last argument of a multi-argument macro.

Property SynthesisTake the following class:

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    @interface MyClass : NSObject {
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        NSString *_myIvar;
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    }
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    @property (copy) NSString *myIvar;
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    @end
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    @implementation MyClass
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    @synthesize myIvar;
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    @end

Nothing wrong with this, right? The ivar declaration and @synthesize are a little redundant in this modern age, but do no harm.

Unfortunately, this code will silently ignore _myIvar and synthesize a new variable called myIvar, without the leading underscore. If you have code that uses the ivar directly, it will see a different value from code that uses the property. Confusion!

The rules for @synthesize variable names are a little weird. If you specify a variable name with @synthesize myIvar = _myIvar;, then of course it uses whatever you specify. If you leave out the variable name, then it synthesizes a variable with the same name as the property. If you leave out @synthesize altogether, then it synthesizes a variable with the same name as the property, but with a leading underscore.

Unless you need to support 32-bit Mac, your best bet these days is to just avoid explicitly declaring backing ivars for properties. Let @synthesize create the variable, and if you get the name wrong, you’ll get a nice compiler error instead of mysterious behavior.

Interrupted System CallsCocoa code usually sticks to higher level constructs, but sometimes it’s useful to drop down a bit and do some POSIX. For example, this code will write some data to a file descriptor:

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    int fd;
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    NSData *data = ...;
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    const char *cursor = [data bytes];
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    NSUInteger remaining = [data length];
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    while(remaining > 0) {
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        ssize_t result = write(fd, cursor, remaining);
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        if(result < 0)
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        {
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            NSLog(@"Failed to write data: %s (%d)", strerror(errno), errno);
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            return;
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        }
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        remaining -= result;
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        cursor += result;
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    }

However, this can fail, and it will fail strangely and intermittently. POSIX calls like this can be interrupted by signals. Even harmless signals handled elsewhere in the app like SIGCHLD or SIGINFO can cause this. SIGCHLD can occur if you’re using NSTask or are otherwise working with subprocesses. When write is interrupted by a signal, it returns -1 and sets errno to EINTR to indicate that the call was interrupted. The above code treats all errors as fatal and will bail out, even though the call just needs to be tried again. The correct code checks for that separately and just retries the call:

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    while(remaining > 0) {
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        ssize_t result = write(fd, cursor, remaining);
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        if(result < 0 && errno == EINTR)
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        {
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            continue;
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        }
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        else if(result < 0)
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        {
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            NSLog(@"Failed to write data: %s (%d)", strerror(errno), errno);
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            return;
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        }
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        remaining -= result;
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        cursor += result;
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    }

String LengthsThe same string, represented differently, can have different lengths. This is a relatively common but incorrect pattern:

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    write(fd, [string UTF8String], [string length]);

The problem is that NSString computes length in terms of UTF-16 code units, while write wants a count of bytes. While the two numbers are equal when the string only contains ASCII (which is why people so frequently get away with writing this incorrect code), they’re no longer equal once the string contains non-ASCII characters such as accented characters. Always compute the length of the same representation you’re manipulating:

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    const char *cStr = [string UTF8String];
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    write(fd, cStr, strlen(cStr));

Casting to BOOLTake this bit of code that just checks to see whether an object pointer is nil:

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    - (BOOL)hasObject
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    {
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        return (BOOL)_object;
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    }

This works… usually. However, roughly 6% of the time, it will return NO even though _object is not nil. What gives?

The BOOL type is, unfortunately, not a boolean. Here’s how it’s defined:

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    typedef signed char BOOL;

This is another bit of unfortunate legacy from the days when C had no boolean type. Cocoa predates C99’s _Bool, so it defines its “boolean” type as a signed char, which is just an 8-bit integer. When you cast a pointer to an integer, you just get the numeric value of that pointer. When you cast a pointer to a small integer, you just get the numeric value of the lower bits of that pointer. When the pointer looks like this:

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    ....110011001110000

The BOOL gets this:

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               01110000

This is not 0, meaning that it evaluates as true, so what’s the problem? The problem is when the pointer looks like this:

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    ....110011000000000

Then the BOOL gets this:

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               00000000

This is 0, also known as NO, even though the pointer wasn’t nil. Oops!

How often does this happen? There are 256 possible values in the BOOL, only one of which is NO, so we’d naively expect it to happen about 1/256 of the time. However, Objective-C objects are allocated aligned, normally to 16 bytes. This means that the bottom four bits of the pointer are always zero (something that tagged pointers takes advantage of) and there are only four bits of freedom in the resulting BOOL. The odds of getting all zeroes there are about 1/16, or about 6%.

To safely implement this method, perform an explicit comparison against nil:

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    - (BOOL)hasObject
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    {
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        return _object != nil;
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    }

If you want to get clever and unreadable, you can also use the ! operator twice. This !! construct is sometimes referred to as C’s “convert to boolean” operator, although it’s just built from parts:

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    - (BOOL)hasObject
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    {
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        return !!_object;
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    }

The first ! produces 1 or 0 depending on whether _object is nil, but backwards. The second ! then puts it right, resulting in 1 if _object is not nil, and 0 if it is.

You should probably stick to the != nil version.

Missing Method ArgumentLet’s say you’re implementing a table view data source. You add this to your class’s methods:

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    - (id)tableView:(NSTableView *) objectValueForTableColumn:(NSTableColumn *)aTableColumn row:(NSInteger)rowIndex
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    {
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        return [dataArray objectAtIndex: rowIndex];
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    }

Then you run your app and NSTableView complains that you haven’t implemented this method. But it’s right there!

As usual, the computer is correct. The computer is your friend.

Look closer. The first parameter is missing. Why does this even compile?

It turns out that Objective-C allows empty selector segments. The above does not declare a method named tableView:objectValueForTableColumn:row: with a missing argument name. It declares a method named tableView::row:, and the first argument is named objectValueForTableColumn. This is a particularly nasty way to typo the name of a method, and if you do it in a context where the compiler can’t warn you about the missing method, you may be trying to debug it for a long time.

ConclusionObjective-C and Cocoa have plenty of pitfalls ready to trap the unwary programmer. The above is just a sampling. However, it’s a good list of things to be careful of.

That’s it for today! Check back next time for more wacky advice. Friday Q&A is driven by user ideas, in case you didn’t already know, so until next time, please send in your ideas for articles!