Question

I noticed that my program was crashing because it was running out of memory. I figured out that this was happening because of this code segment:

DataSet *tempSet = [[DataSet alloc] init];          
tempSet.rightFoot = [NSNumber numberWithDouble:temp1];
tempSet.leftFoot = [NSNumber numberWithDouble:temp2];
[footData addObject:tempSet];   //add dataSet object to the array
[tempSet release];

I read some tutorials about memory management online and was able to figure out that I needed to do this: (notice the added “autoreleases”)

DataSet *tempSet = [[DataSet alloc] init];          
tempSet.rightFoot = [[NSNumber numberWithDouble:temp1] autorelease];
tempSet.leftFoot = [[NSNumber numberWithDouble:temp2] autorelease];
[footData addObject:tempSet];   //add dataSet object to the array
[tempSet release];

I am still confused about why I had to do this. I did not use alloc, new or copy when creating the numberWithDouble.

Does this mean that I would need to add autorelease in this situation as well?:

[subset addObject:[NSNumber numberWithDouble:temp]];

What about this situation?:

tempSet.rightFoot = [NSString stringWithString:@"temp"];

I appreciate any help.

Answer 1

+numberWithDouble

is called a convenience method. Meaning, it replaces the little section of code that would look like this:

[[[NSNumber alloc]initWithDouble:double]autorelease];

Most (if not all) convenience methods are auto release by default, so the OP code with the autoreleases is incorrect, as it drops the retain count to -1.

The equals sign however is equivalent to

[self setRightFoot:[[[NSString alloc]initWithString]autorelease]];

which increments rightFoot’s retain count and requires it to be released elsewhere.

as for the -addObject code, it returns void, so it does not in fact increment the receiver’s retain count, and requires no release of the receiver. The object in the array should already be released by the convenience method for later, which doesn’t matter because the array is now holding “copy” of it.