Got EXC_BAD_ACCESS while printing a binary search tree in Objective - C

Question

I am getting stack-overflow(Thread 1: EXC_BAD_ACCESS (code=2, address=0x7ffeef3ffff8)) while printing the binary search tree. After some inspection, it seems like the root of the tree becomes nil before starting the inOrderTraversalRecursive method. Not sure why the root becomes nil. I am aware of this andthis resources, but they didn't help me so far. Thanks for taking the time to answer my question.

in main.m file

#import <Foundation/Foundation.h>

@interface BSTNode : NSObject
@property (nonatomic, assign) int data;
@property (nonatomic, strong) BSTNode *left;
@property (nonatomic, strong) BSTNode *right;
@end

@implementation BSTNode
@end

@interface BST: NSObject

@property (nonatomic,strong) BSTNode *root;

- (void)insertNode:(int)value;
-(void)inOrderTraversal;
@end

@implementation BST

-(instancetype)init{
    if (self = [super init]) {
        self.root = [self initializeTreeNode];
    }
    return self;
}

- (BSTNode *)initializeTreeNode {
    // By default, |data| is 0, |left| is nil, |right| is nil
    return [[BSTNode alloc] init];
}

- (void)insertNode:(int)value {
    self.root = [self insertNode:_root withData:value];
}

- (BSTNode *)insertNode:(BSTNode *)root withData:(int)data {
    if(!root) {
        root = [self initializeTreeNode];
        root.data = data;
    } else if (root.data >= data) {
        root.left = [self insertNode:root.left withData:data];
    } else {
        root.right = [self insertNode:root.right withData:data];
    }

    return root;
}

-(void)inOrderTraversal {
    [self inOrderTraversalRecursive:self.root];
}

-(void)inOrderTraversalRecursive:(BSTNode *)root {
    // inOrder = left - root - right
    [self inOrderTraversalRecursive:root.left];
    NSLog(@"%d",root.data);
    [self inOrderTraversalRecursive:root.right];
}

@end

inside main.m

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        BST *bst = [BST new];
        [bst insertNode:50];
        [bst insertNode:30];
        [bst insertNode:20];
        [bst insertNode:40];
        [bst insertNode:70];
        [bst insertNode:60];
        [bst insertNode:80];
        [bst inOrderTraversal];
    }
    return 0;
}

PS.

I designed BST and BSTNode classes based on geeksforgeeks java implementation of BST. What is a proper implementation of BST(insert, find, print, delete) in Objective-C? Can't seem to find anywhere.

Answer 1

(Thread 1: EXC_BAD_ACCESS (code=2, address=0x7ffeef3ffff8)) means that you are accessing memory block that has been already deallocated. This means that your object has been deallocated from memory and you can no longer access it. That's why bad access.

Also, it could mean you are holding dangling pointer pointing to memory address that no longer exists.

Answer 2

I was missing the nil check in the traversal function.

-(void)inOrderTraversalRecursive:(BSTNode *)root {
    if (!root) {
        return;
    }
    [self inOrderTraversalRecursive:root.left];
    NSLog(@"%d",root.data);
    [self inOrderTraversalRecursive:root.right];
}