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Editorial Team
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Asked: 2026-06-15T07:48:30+00:00

In Objective C how do I explicitly release an object owned by another object?

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In Objective C how do I explicitly release an object owned by another object? Or perhaps equivalently inform the owning object that it does not own the child object anymore. Consider the following:

- (void) testWithNSString:(NSString *)val
{
  NSData *data = [val dataUsingEncoding:NSASCIIStringEncoding];
  // ...
  // now I want to explicitly release data e.g. due to low memory
}

In [val dataUsingEncoding:NSASCIIStringEncoding] an NSData object is returned and I assume (due to general conventions of ownership) that it will be released, when val is released. This seems acceptable from a non-leaking perspective. But this strategy raises two issues:

  1. What if I want to explicitly release data object immediately e.g. due to low memory?
  2. What if I do not want the caller of testWithNSString to be left with an NSString val object (after the testWithNSString has returned), which suddently has ownership of new objects?
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1 Answer

  1. Editorial Team

    In [val dataUsingEncoding:NSASCIIStringEncoding] an NSData object is returned and I assume (due to general conventions of ownership) that it will be released, when val is released

    This is incorrect. dataUsingEncoding will create a new NSData object that has no relationship with val (as to lifetime). It is an autoreleased object and it will be released at the next cycle of the run loop (approx, we don’t know exactly), unless you retain it somehow.

    What if I want to explicitly release data object immediately e.g. due to low memory?

    If you want the object to be released immediately, and you are using ARC, you can simply assign nil to its pointer:

    data = nil;

    If you are not using ARC, your best option is to wait for the run loop autorelease pool to do its job. Or you can create a local autorelease pool in your method, so that autoreleased objects are drained at the end of the method and not at the next run loop cycle. This will make a difference only in case of long-running methods (or chain of methods), so I don’t know if it would make sense in your case.

    What if I do not want the caller of testWithNSString to be left with an NSString val object (after the testWithNSString has returned), which suddently has ownership of new objects?

    As I said, val has no ownership of data.

    Summing this all up, your data object is autoreleased and it will be pretty soon after your method ends (unless it is a long-running method); so data memory will be recovered in an efficient way and I would not worry about it (unless it is a long-running method, as I said).

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