main.m

#import "NSArray+BlockEnumerationTest.h"

int main(int argc, char *argv[])
{
    NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
    
    //Beginning of Sam Soffes's code
    
    NSArray *array = [[NSArray alloc] initWithObjects:@"foo", @"bar", @"bat", nil];
    __block NSString *string = @"";
    [array enumerateObjectsUsingBlockType1:^(id obj, NSUInteger idx, BOOL *stop) {
        string = [(NSString *)obj stringByAppendingString:string];
    }];
    NSLog(@"string: %@", string); // Crashes here
    [array release];
    
    //Ending of Sam Soffes's code
    
    [pool release];
    
    return 0;
}

NSArray+BlockEnumerationTest.h

@interface NSArray (BlockEnumerationTest)

-(void)enumerateObjectsUsingBlockType1:(void (^)(id, NSUInteger, BOOL *))block;
-(void)enumerateObjectsUsingBlockType2:(void (^)(id, NSUInteger, BOOL *))block;

@end

NSArray+BlockEnumerationTest.m

#import "NSArray+BlockEnumerationTest.h"

@implementation NSArray (BlockEnumerationTest)

-(void)enumerateObjectsUsingBlockType1:(void (^)(id, NSUInteger, BOOL *))block
{
    if(!block) { return; }
    
    for(NSUInteger i = 0; i < self.count; i++)
    {
        BOOL stop = NO;
        
        block([self objectAtIndex:i], i, &stop);
        
        if(stop) { break; }
    }
}

-(void)enumerateObjectsUsingBlockType2:(void (^)(id, NSUInteger, BOOL *))block
{
    if(!block) { return; }
    
    for(NSUInteger i = 0; i < self.count; i++)
    {
        BOOL stop = NO;
        
        NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
        
        block([self objectAtIndex:i], i, &stop);
        
        [pool release];
        
        if(stop) { break; }
    }
}

@end