5

Does objective-c methods support "pass by value"? Or perhaps to be more specific:

  1. Is the default behavior for parameters passed into a method pass-by-reference?
  2. If yes, are there any variations from this in some circumstances - for example if the parameter is just a basic int as opposed to an object? (or is this not relevant in objective-c)
  3. Is there anyway to have a method support pass-by-value for a basic variable such as int?
  4. Is there anyway to have a method support pass-by-value for an object? (I'm assuming no here, but for completeness will ask. Of course one could within the message do the copy yourself, however for this approach I'll consider this not to be something objective-c methods offers you, i.e. rather it was a do-it-yourself)

thanks

Greg
  • 34,042
  • 79
  • 253
  • 454

2 Answers2

8

Objective-C does not support references, at least not in the C++ sense of the term.

All Objective-C objects are allocated on the heap, and therefore all "object variables" must in fact be pointer types. Whether a pointer can be considered to be effectively the equivalent of a reference is open to debate. When talking C++ for example, there are clear semantic differences (otherwise, what's the point...)

So to answer your questions:

  1. No, Objective-C only supports pass-by-value. If you pass an object pointer to a method, you pass the pointer by value - you are not passing a reference.
  2. There is no inherent difference between objects and primitives in this regard, apart from the fact that objects are always referred to by pointer, never by value. You can pass a primitive type pointer in if you like.
  3. Yes. This is always the case. Again, if you pass in a pointer to a primitive, you are passing a pointer by value, not a reference.
  4. You're pretty much bang on the mark with this one, other than the fact that you're passing around pointers, not references.
Mac
  • 14,615
  • 9
  • 62
  • 80
5

  1. No. It's pass-by-value by default, like in C. Except for the fact that for the Objective C class instance references, the value is a reference. So Objective C class instances are passed effectively by reference.

  2. N/A

  3. See 1.

  4. Not really. You can serialize, pass the string, and recreate inside. Or you can have the object store its ivars as a structure and pass that structure by value. Some objects support cloning.

Seva Alekseyev
  • 59,826
  • 25
  • 160
  • 281
  • -1 A pointer emulates a reference, but it's not. The accepted definition is that a variable used as method parameter is **passed-by-reference** when the method code receives a reference to the variable, instead of a copy. This describes the implementation, not the effective results. Objective-C only passes by value, be it pointers or anything else. Even when using `&p` (in eg: the NSError patter) you are changing through a memory access, not using a variable reference. C pointers are not the same as [C++ references](http://stackoverflow.com/a/57492/412916). – Jano Dec 17 '12 at 04:41
  • What does C++ have to do with it? Objective C does not have a reference datatype in the C++ sense of the word. – Seva Alekseyev Dec 17 '12 at 13:53
  • Nothing. I see in your answer that you already understand all this. I only commented and voted -1 for the benefit of whoever visits this page, because it's not the proper term to call reference to a pointer in the context of a pass-by-value/reference discussion. – Jano Dec 17 '12 at 20:37