Question

I’m new to Objective-C and I am having some difficulty with understanding memory management.

So let’s say I have a class and an instance variable of type NSString* that isn’t tied to a property. Would this situation leak memory?

myString = [[NSString alloc] init]; 
//more program stuff
myString = [[NSString alloc] init];

Should I have called [myString release] before I set myString equal to a new object? Will this sort of code leak memory? Do I have to release an object like this every time I have the pointer point to a different object?

Answer 1

First, Apple’s Memory Management Programming Guide is a great place to look for examples and instructions about memory management.

Now to your question. When you call myString = [[NSString alloc] init]; you are reassigning the pointer myString and as such lose access to the original NSString, thus creating a memory leak.

The general rule of thumb here is that for every alloc you should have a release and these must alternate appropriately. If you do

myString = [[NSString alloc] init]; 
// ...
myString = [[NSString alloc] init];
// ...
[myString release];
[myString release];

you are releasing the same instance twice which results in overreleasing and an error BAD-ACCESS. The correct thing to do is

myString = [[NSString alloc] init]; 
// ...
[myString release];
myString = [[NSString alloc] init];
// ...
[myString release];

so that each instance is correctly released.