Question

I encounter an error when I run this program from Chapter 8 of Cocoa Programming for Mac OSX by Aaron Hillegass.
The program binds a tableview to an array controller. In the setEmployees method of the array controller,

-(void)setEmployees:(NSMutableArray *)a
{
    if(a==employees)
        return;
    [a retain];//must add
    [employees release]; //must add
    employees=a;
}

In the book, the two retain and release statements were not included and my program crashes whenever I try to add a new employee. After googling I found these two must-add statements to prevent program crashing.
I do not understand the memory management here. I am assigning a to employees. Why must I retain a if I am not deallocating anything? Why can I release employees before using it in the last assignment statement?

Answer 1

This is the standard pattern for setters using Manual Reference Counting (MRC). Step by step, this is what it does:

-(void)setEmployees:(NSMutableArray *)a 
{ 
    if(a==employees) 
        return;          // The incoming value is the same as the current value, no need to do anything. 
    [a retain];          // Retain the incoming value since we are taking ownership of it
    [employees release]; // Release the current value since we no longer want ownership of it
    employees=a;         // Set the pointer to the incoming value
} 

Under Automatic Reference Counting (ARC) the accessor can be simplified to:

 -(void)setEmployees:(NSMutableArray *)a 
    { 
        if(a==employees) 
            return;          // The incoming value is the same as the current value, no need to do anything. 
        employees=a;         // Set the pointer to the incoming value
    } 

The retains / releases are done for you. You haven’t said what sort of crash you are getting, but it seems like you are using ARC sample code in an MRC project.