EXC_BAD_ACCESS when using recursive block

Question

I\'m trying to create recursion using blocks. It works for a while, but eventually it crashes and gives me a bad access exception. This is my code:

BOOL (^Bl         


        

Answer 1

You seem to be adding squares to the array while traversing the array. I'm speaking of this line:

[processedSquares addObject:square];

Might that have to do with it? You're adding an object while traversing. I'm surprised that this works at all.

Answer 2

You need a __block on Block, change the declaration to:

__block BOOL (^Block)(Square *square, NSMutableArray *processedSquares);
Block = ^(Square *square, NSMutableArray *processedSquares) {

When a variable (Block) is referenced within a block then its current value is copied into the block. In your code Block hasn't yet been given a value, as you are constructing the block in the assignment...

The __block prefix passes the variable by reference - by the time your block makes its recursive call Block has a value, the reference to it is used to obtain that value, and the recursive call is OK.

I don't know why its worked at all for you without the __block - failed straightaway for me. With the modifier however I can recurse to at least a depth of 10,000 - so stack space isn't a problem!

Answer 3

You're liking doing something wrong with the setup -- your Square objects are probably messed up somehow. Here's a complete example that works fine for me, maybe it can help you find your mistake:

#include <stdio.h>
#include <Foundation/Foundation.h>

@interface Square : NSObject
{
  BOOL nuked;
  NSArray *adjacentSquares;
}

@property(nonatomic) BOOL nuked;
@property(nonatomic, retain) NSArray *adjacentSquares;
@end

@implementation Square

@synthesize nuked;
@synthesize adjacentSquares;

@end;

BOOL (^Block)(Square *square, NSMutableArray *processedSquares) = ^(Square *square, NSMutableArray *processedSquares) {
  [processedSquares addObject:square];

  if (square.nuked) {
    return YES; // Found a nuked square, immediately return
  }

  for (Square *adjacentSquare in square.adjacentSquares) {
    if ([processedSquares containsObject:adjacentSquare]) {
      continue; // Prevent infinite recursion
    }

    if (Block(adjacentSquare, processedSquares)) {
      return YES;
    }
  }

  return NO;
};

int main(int argc, char **argv)
{
  NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

  Square *s1, *s2;
  s1 = [[Square alloc] init];
  s2 = [[Square alloc] init];
  s1.adjacentSquares = [NSArray arrayWithObjects:s2, nil];
  s2.adjacentSquares = [NSArray arrayWithObjects:s1, nil];

  __block NSMutableArray *processedSquares = [NSMutableArray array];
  BOOL foundNukedSquare = Block(s1, processedSquares);
  printf("%d\n", foundNukedSquare);

  [s1 release];
  [s2 release];

  [pool release];

  return 0;
}