objective c - On lazy instantiation and convenience methods

Question

Assume you have a Singleton Constants class, instance of which you'd like to use throughout your application.

In someClass, therefore we can reference [Constants instance] someCleverConstant];

Typing this gets old really quick and it would be nice to get a shortcut to the instance.

• In someClass, we can declare @property (nonatomic, weak, readonly) Constants *constants;
• And a getter to the instance

-(Constants*) constants {
  if (constants == nil) 
    constants = [Constants instance];
  return constants;
}

This way in someClass, therefore we can reference constants.someCleverConstant; instead

A few questions on this:

• Is what i described a reasonable approach?
• Is it correct to declare a property weak?
• Is there any performance concerns with what i have described? Would it actually be better to call instance directly?
• Consider a situation where you have 20 classes, each needing it's own pointer to Constants instance. Would this approach work then?

Thank you for your time.

Answer 1

Following @vinceburn I would use the following example for constants and a singleton for more complex structures.

// Constants.h
// Replace PSMyApp for something more useful. e.g. company/name initials followed by app/class

// String example
NSString * const PSMyAppString = @"constantString"; 

// Logically related integers
typedef enum {
   PSMyAppRelatedValuesOne = 0,
   PSMyAppRelatedValuesTwo,
   PSMyAppRelatedValuesThree
} PSMyAppRelatedValues;

// Float example
const CGFloat PSMyAppFloat = 0.3f;

// Integer that has no related values
const NSInteger PSMyAppInteger = 2;

I prefer this over #define as I get auto completion and compiler checking and it fits more naturally with the way Apple does thing in some of the UIKit classes.