EXEC_BAD_ACCESS in UITableView cellForRowAtIndexPath

Question

My UITableView is returning EXEC_BAD_ACCESS , but why!

See this code snippet!

Loading the UITableView works fine, so allXYZArray != nil and is populated!

Then scrolling the tableview to the bottom and back up causes it to crash, as it goes to reload the method cellForRowAtIndexPath

It fails on line:

    "NSLog(@"allXYZArray::count: %i", [allXYZArray count]);"

        (UITableViewCell *)tableView:(UITableView *)theTableView cellForRowAt

IndexPath:(NSIndexPath *)indexPath { 

static NSString *CellIdentifier = @"CellIdentifier";
UITableViewCell *cell = [theTableView dequeueReusableCellWithIdentifier:CellIdentifier];


cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
@try
{ 
if (allXYZArray == nil) {
   NSLog(@"nil");
   allXYZArray = [ToolBox getMergedSortedDictionaries:allXYZGiven SecondDictionary:allXYZSought];
}
NSLog(@"%i", [indexPath row]);
NSLog(@"allXYZArray::count: %i", [allXYZArray count]);

Answer 1

EXC_BAD_ACCESS means that your program is trying to access a memory address that is invalid or otherwise inaccessible from your process. This most commonly happens when you try send a message to an object that has already been dealloced. So the first step in debugging EXC_BAD_ACCESS is to figure out which object your program was trying to send a message to when the crash happened. Often the answer isn't obvious, in which case, NSZombieEnabled is a great tool for identifying which line of code caused the crash.

In your case you've already determined that the crash happens when you call [allXYZArray count] , making allXYZArray our prime suspect. This object is being returned from +[ToolBox getMergedSortedDictionaries:SecondDictionary:] , so it's likely that your bug is in the implementation of that method. I would guess that it's returning an object that has already been released instead of autoreleased, as prescribed by the Memory Management Programming Guide for Cocoa . (This is one of the most important documents in the SDK, by the way. I recommend rereading it once a month until its policies and techniques become second nature.)

Answer 2

Ok, reusing a cell, does not guarantee that the cell will be initialized properly:

UITableViewCell *cell = [theTableView dequeueReusableCellWithIdentifier:CellIdentifier];

cell will sometimes be null (especially the first time I guess).

Check cell for null and if so, initialize it properly.

if (cell == nil)
   cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];