Question

Say I have a class method like

+ (double)function:(id)param1 :(id)param2
{
   // I want to memoize this like...
   static NSMutableDictionary* cache = nil;
   //
   // test if (param1,param2) is in cache and return cached value, etc. etc
   //
}

Thanks!!

Answer 1

If you want to create the cache once and check against it, I generally use an +initialize method. This method is called before the first message sent to the class, so the cache would be created before +function:: (which, by the way, is a terrible selector name) could be called. In this case, I usually declare the cache variable in the .m file, but declaring it in the method definition may also work.

---

Edit: Adding an example at request of OP:

// MyClass.m

static NSMutableDictionary* cache;

+ (void) initialize {
    cache = [[NSMutableDictionary alloc] init];
}

+ (double) cachedValueForParam1:(id)param1 param2:(id)param2 {
    // Test if (param1,param2) is in cache and return cached value.
}

Obviously, if a value doesn’t exist in the cache, you should have some code that adds the value. Also, I have no idea how you intend to combine param1 and param2 as the key for the cache, or how you’ll store the value. (Perhaps +[NSNumber numberWithDouble:] and -[NSNumber doubleValue]?) You’ll want to make sure you understand dictionary lookups before implementing such a strategy.