Objective c — Update parameter in block - objective-c

Question

I was doing some tinkering with tree traversals (which I have solved in a much more straightforward way) but I have come across an issue in the following piece of Objective C logic:
- (NSString *)someWrapperFunction
{
NSString *result = #"";
NSString *(^appendBlock)(int, NSString **) = ^NSString *(int a, NSString **adder){
if (a == 0)
{
// base case
return #"";
}
NSLog(#"%d", a);
*adder = [*adder stringByAppendingFormat:#"-%d-", a];
NSLog(#"adder: %#", *adder);
return [*adder stringByAppendingString:appendBlock(a-1, adder)];
};
appendBlock(5, &result);
return result;
}
Basically, I want to create a block of code that concatenates numbers into the given string (adder). The result should be: "-5--4--3--2--1-".
I get a segmentation fault with the above code but with some other code that I wrote for the tree traversal, the adder string was essentially not getting updated. Any pointers to what I am doing wrong here? (Is it possible that the variable that is being updated by the inner block (inside recursion) is disallowed as it is already being occupied by the outer block OR is it just that NSString is non-mutable data type?)
In any case, I want to keep the design of the function the same; how would I solve this problem (using c/objective)?

Answer 1

After some searching and experimenting I found a way to fix this.
There is no reason to be using a double-pointer for your adder parameter in the block. Just use a regular pointer and update your code accordingly.
The error is coming from the fact that inside of the block, appendBlock is NULL and you end up dereferencing the NULL pointer trying to call it.
Here's an updated version that works:
- (NSString *)someWrapperFunction
{
NSString *result = #"";
NSString *(^appendBlock)(int, NSString *);
__block __weak NSString *(^weakBlock)(int, NSString *);
weakBlock = appendBlock = ^NSString *(int a, NSString *adder){
NSString *(^innerBlock)(int, NSString *) = weakBlock;
if (a == 0)
{
// base case
return #"";
}
NSLog(#"%d", a);
adder = [adder stringByAppendingFormat:#"-%d-", a];
NSLog(#"adder: %#", adder);
// Split this update to make it easier to debug.
NSString *update = innerBlock(a-1, adder);
return [adder stringByAppendingString:update];
};
appendBlock(5, result);
return result;
}
Output: "-5--4--3--2--1-"
This update is rewritten for point #1 (which really has nothing to do with your original issue.
To solve point #2 this update creates the original appendBlock variable as well as a new __block __weak weakBlock reference to the same block. And then inside the block, a new (strong) block pointer is created to reference the weak block pointer. Without the use of the weak pointer, the code works but causes a warning.