Ruby » Ruby master

Feature #16471

Updated by nobu (Nobuyoshi Nakada) over 4 years ago

I'd like to request two features for `WeakRef`. WeakRef. I'll explain what I want, then provide a real world use case. 

 First, add the ability to pull the original object out of the `WeakRef` WeakRef object, something like this: 

 ```ruby 
 

     myhash = {} 
 
     myhash = WeakRef.new(myhash) 
 
     myhash = myhash.original_object() 
 ``` 

 Second, add a callback feature for when a WeakRef's object is being purged by GC. It would work something like this: 

 ```ruby 
 

     wr = WeakRef.new(myobject) 

 
    
     wr.on_garbage() do |wr| 
     
         puts 'trashing' 
 
     end 

 
    
     # time goes by... 
 
     # myobject goes out of scope 
 
     # outputs "trashing" 
 ``` 

 Here's the specific use case I would want it for. I'm developing a database system which includes a class called `Node`. Node. A `Node` Node object holds a reference to a database handle and the primary key of a record in that database. It also has methods for getting and setting values in that record. So a simplified version looks something like this: 

 ```ruby 
 

     class Node 
     
         attr_reader :dbh 
     
         attr_reader :pk 
    
     
        
         def initialize(dbh, pk) 
         
             @dbh = dbh 
         
             @pk = pk 
     
         end 
    
     
        
         def set(fieldname, value) 
         
             # a bunch of SQL to set the value 
     
         end 
    
     
        
         def get(fieldname) 
         
             # a bunch of SQL to get the value 
         end 
     end 
 end 

 
    
     node = Node.new dbh, 'abc' 
 
     node.set 'name', 'Fred' 
 ``` 

 The database object will have a node method, so you would usually get nodes like this: 

 ```ruby 
 

     node = dbh.node('abc') 
 ``` 

 Make sense so far? It's a pretty simple concept. It works, but I'd like to make a small improvement. (Whether or not it's actually an improvement is a judgement call... I expect some disagreement on this point. But work with me here.) 

 I'd like the database object to keep a cache of `Node` Node objects. However, the database doesn't keep those node objects alive: they can fall out of scope and get purged by the GC. However, if that cache is never purged of dead objects, it just grows bigger and bigger. I could occasionally just run a routine to work through the cache and purge dead references, but that seems very inefficient. It would be better to just have them purged as they die. 

 So I could implement it something like this: 

 ```ruby 
 

     class DataBase 
     
         attr_reader :cache 
    
     
        
         def initialize 
         
             @cache = {} 
     
         end 
    
     
        
         def node(pk) 
         
             if @cache[pk] 
             
                 # Here's where we need to get at the original object 
             
                 return @cache[pk].original_object 
         
             else 
             
                 new_node = Node.new(self, pk) 
            
             
                
                 @cache[pk] = WeakRef.new(new_node) 
            
             
                
                 # Here's where we set the callback 
             
                 @cache[pk].on_garbage do |wr| 
                 
                     wr.db.cache.delete wr.pk 
             
                 end 
            
             
                
                 return new_node 
             end 
         end 
     end 
 end 
 ``` 

 So when a `Node` Node object is garbage collected, it's deleted from the cache. The cache stays clean of dead objects. 

 I'll be interested to hear what you think of this idea and how difficult it would be to implement it.

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