Objective-C - Understanding "release"

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July 18th, 2010, 04:45 PM

thomashw

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Other Language - Objective-C - Understanding "release"

I've bolded the part of the code I don't fully understand.

Won't sum = sum2 just overwrite the old sum? So why do you need to release it beforehand?
Why isn't sum2 released at the end of the program? Should it be?

Code:

#import “Fraction.h”

int main (int argc, char *argv[])
{

NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
Fraction *aFraction = [[Fraction alloc] init]; Fraction *sum = [[Fraction alloc] init], *sum2; int i, n, pow2;

[sum setTo: 0 over: 1]; // set 1st fraction to 0

NSLog (@”Enter your value for n:”);
scanf (“%i”, &n);
pow2 = 2;

for (i = 1; i <= n; ++i) {
     [aFraction setTo: 1 over: pow2];
     sum2 = [sum add: aFraction];
     [sum release]; // release previous sum
     sum = sum2;
     pow2 *= 2;
}

NSLog (@”After %i iterations, the sum is %g”, n, [sum convertToNum]); [aFraction release];
[sum release];
[pool drain];

return 0;
}

July 19th, 2010, 11:52 PM

thomashw

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Bump.

July 21st, 2010, 04:15 PM

OmegaZero

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You'll probably get more/better answers in the C forum.

I don't know Objective-C, but (assuming alloc/release == new/delete) it sounds like you're confusing references with values. "sum" and "sum2" aren't actual Fraction objects, but are just the address of Fraction objects. So "sum = sum2" only makes "sum" and "sum2" refer to the same Fraction object. The one "sum" originally referenced would still be out there in memory (hence the release). You don't need to release "sum2" at the end of the program since after the loop ends both "sum" and "sum2" refer to the same Fraction object, calling release using "sum2" would be an error since the Fraction object it refers to was already released when you called release using "sum".

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August 23rd, 2010, 04:53 PM

TheOtherDino

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Quote:

Originally Posted by thomashw

I've bolded the part of the code I don't fully understand.

Won't sum = sum2 just overwrite the old sum? So why do you need to release it beforehand?
Why isn't sum2 released at the end of the program? Should it be?

Code:

#import “Fraction.h”

int main (int argc, char *argv[])
{

NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
Fraction *aFraction = [[Fraction alloc] init]; Fraction *sum = [[Fraction alloc] init], *sum2; int i, n, pow2;

[sum setTo: 0 over: 1]; // set 1st fraction to 0

NSLog (@”Enter your value for n:”);
scanf (“%i”, &n);
pow2 = 2;

for (i = 1; i <= n; ++i) {
     [aFraction setTo: 1 over: pow2];
     sum2 = [sum add: aFraction];
     [sum release]; // release previous sum
     sum = sum2;
     pow2 *= 2;
}

NSLog (@”After %i iterations, the sum is %g”, n, [sum convertToNum]); [aFraction release];
[sum release];
[pool drain];

return 0;
}

Since sum and sum2 are both pointers to instances of Fractions, all

Code:

sum = sum2 ;

will do is overwrite the pointer contained in sum, thus causing a memory leak to the instance that sum used to point to. You need to release it before the assignment of sum2. Remember, the setTo method in Fraction.h will return a pointer to a new instance of Fraction.

sum2 could have been released at the end of the program, as both sum and sum2 point to the same instance.

Other Language - Objective-C - Understanding "release"

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