Tutorial :NSMutableArray with only a particular type of objects


is it possible to specify that a NSMutableArray can only contain a certain type of objects. For example, if I want to store only this kind of objects :

@interface MyObject : NSObject {      UInt8 value;   }  

In order to be able to use the instance variable like this :

- (void)myMethod:(NSMutableArray *)myArray{      for (id myObject in myArray){          [self otherMethod:myObject.value];      }  }  

because I'm getting this error :

request for member 'value' in something not a structure or union  

Thank you for your help


You are getting that error, because for as far as Objective-C is concerned, myObject is of the non-type id, which doesn't support the value property. To make Objective-C aware of the fact it's always dealing with a MyObject in this loop, you'll have to tell it the myObject object is an instance of MyObject.

for (MyObject *myObject in myArray) {  

Also, you have to make sure the value ivar is accessible using dot-notation by implementing getter and setter methods for it. You can do this yourself by implementing -value and -setValue:, or you can use @property and @synthesize to let Objective-C do this.


It sounds like you're coming from a Java/C# type background where limits can be imposed on collections.

Collections in Cocoa don't follow that pattern. There is no way to set a restriction on what type of objects can be inserted (unless you write a wrapper class that enforces this).

Objective-C, by design, follows the "if it walks like a duck and it quacks like a duck, then it most probably is a duck" philosophy. That is to say that rather than checking whether an object is a particular type, you should be checking whether it can do what you want it to do regardless of its type.

You can do this using respondsToSelector:.

Finally, your problem isn't actually related to the fact that the array has no restrictions. Your object doesn't appear to declare the instance variable value as a property, or expose any accessor methods for it.

This is why you're seeing the error when you try myObject.value. That syntax in Objective-C is how you access properties.

The default scope for instance variables in Objective-C is @protected, which means anything outside your class can't access them without going through an accessor method of some kind.

You need to declare and define the methods - (UInt8)value and - (void)setValue:(UInt8)aValue and use them.

Alternatively, you could declare it as a property.


Objective-C doesn't work like that. You need to use [myObject value] (which will work irrespective of the kind of object, as long as it responds to -[value]. If you only want one type of objects in it, insert only that type of objects.


You would have to write a wrapper-class for the NSMutableArray, see for example this question.


Subclass NSMutableArray and override methods that mediate the addition of objects to the array. You would check the object type in these overridden methods, only calling [super addObject:xyz] if the type is accepted.


maybe you can use protocol:

@protocol Person <NSObject>  @end    @interface Person : NSObject <Person>  @end  

to use:

NSArray<Person>*  personArray;  

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