macOS question about releasing objects created using instance method

[klaxamazoo]

Hi,

I'm going through Kochran's "Programming in Objective-C 2.0" and I have a question about releasing objects allocated from an object's instance method as shown in Program 7.6 on page 153-154.

I'm trying to figure out how sum2 is released where only sum is explicitly released in the for loop and at the end of the main function.

I notice that sum is released just before the sum = sum2 statement and I was wondering exactly what the = sign does. I thought that it set the instance variables in the sum object equal to those in the sum2 object.

Clearly I'm wrong but I would like some help understanding exactly what occurs when I write sum = sum2.

Here is the Main Code

#import "Fraction.h"

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    Fraction *aFraction = [[Fraction alloc] init];
	Fraction *sum		= [[Fraction alloc] init];
	Fraction *sum2;		// sum2 is allocated by "add" method
	int i, n, pow2;
	
	[sum setNumerator: 0 setDenominator: 1]; // initilize sum to 0
	
	NSLog(@"Enter your falue for n");
	scanf ("%i", &n);
	
	pow2 = 2;
	for (i = 1; i <= n; ++i) {
		[aFraction setNumerator: 1 setDenominator: pow2];
		sum2 = [sum add: aFraction];
		[sum release];						// release previous sum
		
		sum = sum2;
		pow2 *= 2;
	}
	
	NSLog(@"After %i iterations, the sum is %g", n, [sum convertToNumber]);
	[aFraction	release];
	[sum release];
	//[sum2 release];
	
	
    [pool drain];
    return 0;
}

 

[rdyornot]

sum is a pointer to a fraction object.
sum2 = sum assigns sum2 pointer to point at the same object / same place in memory.

for example if sum and sum2 are both pointers (as they are in your code):
sum2 = sum;
sum = 5;
sum2 will now also = 5.
sum = 8;
sum2 will now also = 8 etc...

does this help? i'm rather new as well so my explanations may not be as clear as they need to be.

 

[klaxamazoo]

sum is a pointer to a fraction object.
sum2 = sum assigns sum2 pointer to point at the same object / same place in memory.

for example if sum and sum2 are both pointers (as they are in your code):
sum2 = sum;
sum = 5;
sum2 will now also = 5.
sum = 8;
sum2 will now also = 8 etc...

does this help? i'm rather new as well so my explanations may not be as clear as they need to be.

Since sum2 and sum are both pointers to objects and by setting sum2 = sum; they are now pointing at the same object. I assume that means that releasing one releases the other? Will I have any memory leaks because I only release one pointer?

It looks like I'll have to reread the section on pointers.

What do I do if I want to copy the values of sum2 to sum?

 

[chown33]

sum is a pointer to a fraction object.
sum2 = sum assigns sum2 pointer to point at the same object / same place in memory.

for example if sum and sum2 are both pointers (as they are in your code):
sum2 = sum;
sum = 5;
sum2 will now also = 5.
sum = 8;
sum2 will now also = 8 etc...

does this help? i'm rather new as well so my explanations may not be as clear as they need to be.

This is wrong.

One reason it's wrong is because neither 5 nor 8 are Fraction objects.

Another reason it's wrong is that assigning a pointer to a variable, such as 'sum = 5', has no effect at all on any other variables, such as sum2. So your assertions about sum2 are wrong.

If you had the same pointer assigned to both variables, and changed the thing pointed at such as by sending a message that changes the object's numerator or denominator, then your assertions would be right. But that's not what you're doing.

 

[Catfish_Man]

Since sum2 and sum are both pointers to objects and by setting sum2 = sum; they are now pointing at the same object. I assume that means that releasing one releases the other?

Yes. To phrase it a little more accurately, there's only one, and releasing it via either pointer releases it.

Will I have any memory leaks because I only release one pointer?

Looks ok to me.

It looks like I'll have to reread the section on pointers.

http://boredzo.org/pointers/ is a very thorough guide a friend of mine wrote.

What do I do if I want to copy the values of sum2 to sum?

http://developer.apple.com/mac/libr...s/NSCopying_Protocol/Reference/Reference.html

(Roughly speaking, implement a -copy method on Fraction that allocates a new Fraction and sets it values to be equal to the original's.
More accurately, implement -copyWithZone: and ignore the zone argument. -copy will call -copyWithZone: )

 

[klaxamazoo]

Thanks a lot for everybody's help. I'm in grad school and am trying to transition from the Matlab and Fortan to a more flexible programming language. It has been a lot of fun so far and Kochran's book really helps a lot.

Yes. To phrase it a little more accurately, there's only one, and releasing it via either pointer releases it.


Looks ok to me.


http://boredzo.org/pointers/ is a very thorough guide a friend of mine wrote.



http://developer.apple.com/mac/libr...s/NSCopying_Protocol/Reference/Reference.html

(Roughly speaking, implement a -copy method on Fraction that allocates a new Fraction and sets it values to be equal to the original's.
More accurately, implement -copyWithZone: and ignore the zone argument. -copy will call -copyWithZone: )