Is Block_copy recursive?

Question

I have some code that essentially boils down to this:

-(void)doSomethingWithBlock:(BlockTypedef)block
{
    [Foo doSomethingElseWithBlock:^() {
        block         


        

Answer 1

The inner Block_copy() isn't really relevant here. What you want to keep track of is whether a given block lives on the stack or on the heap. Consider this code based on your example:

@interface Foo : NSObject
@end

@implementation Foo

typedef void(^BlockTypedef)(void);

+(void)doSomethingElseWithBlock:(BlockTypedef)block
{
  NSLog(@"block=%@", block);
  BlockTypedef myBlock = Block_copy(block);
  NSLog(@"myBlock=%@", myBlock);
  myBlock();
  Block_release(myBlock);
}

+(void)doSomethingWithBlock:(BlockTypedef)block
{
  [Foo doSomethingElseWithBlock:^() {
    block();
  }];
}
@end

int main (int argc, const char * argv[])
{
  NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
  int i = 3;
  BlockTypedef block = ^{ printf("i=%d\n", i); };
  NSLog(@"block=%@", block);
  [Foo doSomethingWithBlock:block];
  block();
  NSLog(@"block=%@", block);
  [pool drain];
  return 0;
}

This should be ok, but block and myblock are different kinds of blocks. block is a stack block and has the scope of the calling stack. It will exist until main() exits. myblock is a malloc (heap) block, and will exist until it is released. You need to make sure that you don't try to take a non-copied reference to block and use it after the stack is done. You can't stick block in an ivar without copying it.

Joachim Bengtsson has the best write-up of this that I know of. @bbum has also written about it. (If bbum wanders in here and says I'm an idiot about this, then listen to him, but I think I'm right here.)