Objective C instance variable initialization in a method

Question

Did any body get this issue? If I need an instance variable, not as a property, and initialize this variable in a method, then when I need it, it is already released. It happens for autoreleased objects. What is the reason for this?

Usually instance variable should have the whole lifetime of the class object. But it seems if the variable is local to a function, and its a autorelease object, it is released when the function exits.

MyClass.h

@interface MyClass:UIViewController {
  NSDate * date;
}

MyClass.m

@implementation MyClass {

- (void) anInit {
  date = [NSDate date];
}

- (void) useDate {
  NSLog (@"%@", date); 
// here date is already release, and get bad access.
}

}

Tom Jefferys · Accepted Answer · 2011-06-28 13:11:28Z

4

You need to retain date.

An autoreleased object will be released when the autorelease pool is next drained. When this happens has nothing to do with the lifecycle of your object.

Your implementation should look like this:

@implementation MyClass {

    - (void) anInit {
      date = [[NSDate date] retain];  // or [[NSDate alloc] init]
    }

    - (void) useDate {
      NSLog (@"%@", date); 
    }

    - (void) dealloc {
        [date release];
        [super dealloc];
    }

}

answered Jun 28, 2011 at 13:11

Tom Jefferys

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2 Comments

karim Over a year ago

Should not the autorelease pool check the references to the object? 'date' is a class level variable, and can be used by other methods/functions in the class. Not it?

Tom Jefferys Over a year ago

An object does not know what other objects have references to it, and the autorelease pool has no way of finding this out either. All it does when it's drained is blindly decrement the reference count of each object it's holding. If that objects reference count reaches zero it is then deallocated, irrespective of whether other objects still have references to it.

Joe · Accepted Answer · 2011-06-28 13:18:13Z

2

[NSDate date] is a Convenience Constructor and is autoreleased, you need to add a retain call. Also make sure anInit is only called once or you will create a memory leak without calling [date release] first.

- (void) anInit {
  date = [[NSDate date] retain];
}

edited Jun 28, 2011 at 13:18

answered Jun 28, 2011 at 13:11

Joe

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