Declaring static variables in cocoa

Question

I want to have a static variable in Cocoa.

After looking at How do I declare class-level properties in Objective-C?, I am unclear whether there is anything wrong with what I have always done so far, i.e.

// Foo.m
static NSString* id;
@interface Foo ()

instead of

// Foo.h
@interface Foo {
}

+(NSString*) id;

// Foo.m
+(NSString*) id
{
  static NSString* fooId = nil;

  if (fooId == nil)
  {
    // create id
  }

  return fooId;
}

Obviously, the second approach offers an opportunity for initializing the id. But if I initialize the id myself somewhere else in the code, within, say a getter:

-(NSString*) getId
{
    if (id==nil) {
      id =  ... // init goes here
    }
    return id;
}

Then is there anything wrong with the simple static declaration approach as opposed to the more complex class function approach? What am I missing?

Answer 1

First, what you are asking for is a global variable, a static is similar but a little bit different...

Putting a static declaration outside of any @interface in a header (.h) file will create a different variable in each implementation (.m) file you include the header in - not what you want in this case.

So static on a declaration creates a variable whose lifetime is that of the whole application execution but which is only visible within the compilation unit (e.g. the implementation file) in which it appears - either directly or via inclusion.

To create a global variable visible everywhere you need to use extern in the header:

extern NSString *id;

and in your implementation repeat the declaration without the extern:

NSString *id;

As to what is wrong with global variable vs. class methods, that is a question on program design and maintainability. Here are just a few points to consider:

• With a method the value cannot be changed unless you provide a setter method as well as getter method. Variables are always read-write.
• Namespace pollution: a class method is only valid when paired with its class name ([YourClass id]); the variable name is valid everywhere it's included simply as id; that both pollutes the name space and loses the connection between id and YourClass - which leads us to...
• Encapsulation: globals variables break strong encapsulation, and encapsulation aids program design and maintenance - this is a big topic.

That said, there can be a time and a place for globals, sometimes...

After question updated

A static variable declared in the implementation is effectively a "class variable" - a variable shared by all instances of the class.

The pros'n'cons of a class variable vs. setter & getter class methods are exactly the same as the pros'n'cons of an instance variable vs. properties & setter/getter instance methods.

Class setters/getters allow for validation and other logic to be executed on each read/write; and localization of memory management - in short the abstraction and encapsulation benefits of any method.

Therefore whether you use a variable or a setter/getter depends on your application. It is the same question as whether you use an instance variable or setter/getter/property.