Objective-C Blocks Quiz

Do you really know how blocks work in Objective-C? Take this quiz to find out.

All of these examples have been verified with this version of LLVM:

Apple clang version 4.1 (tags/Apple/clang-421.11.66) (based on LLVM 3.1svn)
Target: x86_64-apple-darwin11.4.2
Thread model: posix

Example A

void exampleA() {
  char a = 'A';
  ^{
    printf("%c\n", a);
  }();
}

This example

Example B

void exampleB_addBlockToArray(NSMutableArray *array) {
  char b = 'B';
  [array addObject:^{
    printf("%c\n", b);
  }];
}

void exampleB() {
  NSMutableArray *array = [NSMutableArray array];
  exampleB_addBlockToArray(array);
  void (^block)() = [array objectAtIndex:0];
  block();
}

This example

Example C

void exampleC_addBlockToArray(NSMutableArray *array) {
  [array addObject:^{
    printf("C\n");
  }];
}

void exampleC() {
  NSMutableArray *array = [NSMutableArray array];
  exampleC_addBlockToArray(array);
  void (^block)() = [array objectAtIndex:0];
  block();
}

This example

Example D

typedef void (^dBlock)();

dBlock exampleD_getBlock() {
  char d = 'D';
  return ^{
    printf("%c\n", d);
  };
}

void exampleD() {
  exampleD_getBlock()();
}

This example

Example E

typedef void (^eBlock)();

eBlock exampleE_getBlock() {
  char e = 'E';
  void (^block)() = ^{
    printf("%c\n", e);
  };
  return block;
}

void exampleE() {
  eBlock block = exampleE_getBlock();
  block();
}

This example

Conclusions

So, what’s the point of all this? The point is always use ARC. With ARC, blocks pretty much always work correctly. If you’re not using ARC, you better defensively block = [[block copy] autorelease] any time a block outlives the stack frame where it is declared. That will force it to be copied to the heap as an NSMallocBlock.

Haha! No, of course it’s not that simple. According to Apple:

Blocks “just work” when you pass blocks up the stack in ARC mode, such as in a return. You don’t have to call Block Copy any more. You still need to use [^{} copy] when passing “down” the stack into arrayWithObjects: and other methods that do a retain.

But one of the LLVM maintainers later said:

We consider this to be a compiler bug, and it has been fixed for months in the open-source clang repository. What that means for any hypothetical future Xcode release, I cannot say. :)

So, hopefully Apple was describing a workaround for bugs that existed at the time their guide was written, and everything should work smoothly with ARC and LLVM in the future. But watch out. ;)

Bryan Klimt
February 5, 2013
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