CSC/ECE 517 Fall 2012/ch1 1w22 an

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Introduction<ref>http://rubylearning.com/satishtalim/ruby_method_missing.html</ref>

A method that belongs to a class is called by creating an object of the class and passing the method name to the object as a message. The object then looks up the method lookup path and tries to match the called method with the defined methods in the class. On success, the method is executed and the result is returned.

If the object does not find a match in its method lookup, in normal circumstances the NoMethodError Exception is raised .

In cases where the user wants to handle the methods which are not defined but are still called, “method_missing” can be defined and the user can handle the methods as he/she sees fit.

Format for Defining method_missing

=> def method_missing(m,*args,&block)

(i) m-> accepts the symbol/name of the undefined method (ii) *args-> accepts the array of arguments passed in the method call (iii) &block->accepts a block passed to the method

Ruby Method Lookup Flow

When the object of a class receives a method name to be executed, the following steps are carried out for matching and executing the method:

  • First, the object looks in its own instance methods.
  • Second, it looks in the list of instance methods that all objects of that class share.
  • Third, in each of the included modules of that class, in reverse order of inclusion.
  • Fourth, it looks in that class’s superclass.
  • Fifth, in the superclass’s included modules, all the way up until it reaches the class Object.
  • Sixth, if it still can’t find a method, the very last place it looks is in the Kernel module, included in the class Object.
  • Finally, it calls method_missing (if defined in the class), else throws up the NOMethodError exception.

This entire tracing that the object does is the method lookup path.

Examples

Calling Defined and Undefined Methods

class A		// creating a class 'A'
def say		// defining a method 'say'
puts " say Hi " // body of method say
end
end

Creating the object of the class 

 a=A.new	       // object of the class
 => #<A:0x2a082e0>     //object id

Calling the defined method 

a.say                  // defined method
=> say Hi	       // returned result

Calling the undefined method 

a.sayhi                // undefined method sayhi
NoMethodError: undefined method `sayhi' for #<A:0x2a082e0>   // the NoMethodError is raised

method_missing Implementation<ref>http://www.thirdbit.net/articles/2007/08/01/10-things-you-should-know-about-method_missing/</ref>

class A
def say
puts " say hi "
end
def method_missing(m,*args,&block)	// defining method_missing
puts " This method does not exist"	// body of method_missing
end
end

Calling a method that is not defined 

a=A.new
a.sayhi                                // calling the undefined method sayhi with no arguments
=> This method does not exist		// this result returned when method_missing is executed

When the object 'a' traces its method lookup path for a matching method 'sayhi', upon failure it resorts to method_missing and the body of method_missing is executed.


Sometimes when a class has many methods that do generally the same kinds of functionality, and the programmer is not sure in advance which methods the user will call since there are so many of them, and all of them are similar, writing code for all of the methods seems futile. In these situations method_missing can be defined to take care of these cases. The below 'Generic Handler' example implements this.

Passing Parameters to an Undefined Method Call

class A
def add(a,b)
a+b
end
def method_missing(name,*args,&block)    // the method_missing is defined and the *args parameter accepts all the parameters passed during 								                                 
                                            the method call   
puts “You have typed the method name wrong and these were the parameters passed ; #{args[0]}, #{args[1]}”								
end                         			
end

The passed parameters are stored in the array 'args' and can be accessed like a normal array


Calling the defined method 

a.add(1,2)		        // calling the defined method add and passing the parameters (1,2)
=> 3                           // result

Calling the undefined method 

a.adds(4,2) 			// calling the undefined method adds and passing the parameter (4,2)
=> You have typed the method name wrong and these were the parameters passed; 4, 2

The user made a genuine mistake by typing 'adds', but this method is not defined. When the 'adds' method with parameters is called, the object 'a' tries to match the method in the method lookup path. Upon failure it invokes method_missing, the args are passed, stored in the array 'args' and the body of method_missing is executed.

Converting Numbers from Roman Representation to Integer Representation<ref>http://courses.ncsu.edu/csc517/common/lectures/notes/wk2.pdf</ref>

class Roman
DIGITS = {'I'=>1,'V'=>5,'X'=>10,'L'=>50,'C'=>100,'D'=>500,'M'=>1000,}
def roman_to_integer(roman_string)
last = nil
roman_string.to_s.upcase.split(//).reverse.inject(0) do
|memo, digit|
if digit_value = DIGITS[digit]
if last && last > digit_value
memo -= digit_value
else
memo += digit_value
end
last = digit_value
end
memo
end
end

def method_missing(method)
str=method.id2name
roman_to_integer(str)
end
end


Calling the undefined methods 

r= Roman.new
r.vii
r.xxix
r.xxiv
r.xxvi

The Output 

=> 7
=> 29
=> 24

method_missing to Log Method Calls<ref>http://expertiza.csc.ncsu.edu/wiki/index.php/CSC/ECE_517_Fall_2007/wiki1b_2_22</ref>

Another application that makes use of method_missing could be a simple logger used for debugging purposes. Many times, it may be required to log the trace of called methods and provide information such as: called method-name, arguments, return type. It can be tedious to repeat this part of code in every method. A simple solution to this problem can be obtained using method_missing as: 

 class SimpleCallLogger
 def initialize(o)
 @obj = o
 end
 def method_missing(methodname, *args)
 puts "called: #{methodname}(#{args})"
 a = @obj.send(methodname, *args)
 puts "\t-> returned: #{a}"
 return a
 end
 end

This program makes use of method_missing in a way that it wraps around called method to output the logging information on entry and on exit, it logs the return type. Further, method_missing intercepts the method call and forward it to internal object with ‘send’ method of ruby. Hence, this use of method_missing acts as wrapper.

Generic Handler

class NoBar
def method_missing(methodname, *args)
define_method(:bar) if "bar" == methodname.to_s
define_method(:nobar) if "nobar" == methodname.to_s
end
end

This is an example of using method_missing as a generic handler to handle when a calling method is not exist. You can use missing_method to dynamically create a method at a runtime.

Advantages of method_missing

  • In addition to specifying the error messages for the undefined methods, method_missing provides a more dynamic behavior in the programming environment.
  • If we are unfamiliar with the usage of the objects we created, then using method_missing is a good technique.
  • Handles problems at runtime.
  • Define's a generic method_missing and handle's any undefined method, a big advantage over Java. In Java, when you call an undefined method, the program will not compile.
  • method_missing falls under the general technique of meta-programming. Employ meta-programming in missing_function to write an another function to handle the call.

Disadvantages of method_missing

  • Slower than conventional method lookup. Simple tests indicate that method dispatch with method_missing is at least two to three times as expensive in time as conventional dispatch.
  • Since the methods being called never actually exist—they are just intercepted at the last step of the method lookup process—they cannot be documented or introspected as conventional methods can.
  • method_missing restricts compatibility with future versions of an API. Introducing new methods in a future API version can break users' expectations.

Key Points

  • In the following example, if within method_missing() we define an undefined method.
class A
@@i = 0
def method_missing(method_id)
puts "In Method Missing #{@@i}"
@@i += 1
self.fun
end
end

a = A.new
a.foo

Output 

The result is a 'stack level too deep' error.

When the 'foo' method is called, after no method match the method_missing is run and this block has a method “self.fun” that is undefined. Here when the program execution encounters 'self.fun' it once again calls method_missing. This goes on in an endless loop till the stack memory is full.

  • Ruby knows method_missing( ) exists, because it's a private instance method of 'BasicObject' that every object inherits. The BasicObject#method_missing( ) responds by raising the NoMethodError. Overriding this method_missing( ) allows you to call methods that don't really exist.
  • If method_missing is only looking for certain method names, don't forget to call the super keyword if you haven't found what you're looking for, so that the other superclass' method_missing can handle it.
  • obj.respond_to? function returns 'true' if the obj responds to the given method. So if you want to know whether your class will respond to a function you can use respond_to? to know the answer. But if method_missing() is used, the output may not be what you expect.

class A 

def method_missing(method_id)
puts "In method_missing"
end
end

a = A.new
puts a.respond_to?(:foo)
a.foo

Output 

false
In method_missing

Similar functionality in other languages

Method missing, one of the dynamic features of Ruby, is not a feature that is unique to Ruby. It exists in Smalltalk, Python, Groovy, some Javascripts and most CLOS (Common Lisp Object System)extensions. In this section we look at the few such similar implementations in other languages. The table below gives different ways the functionality related to method_missing is handled in other languages.<ref>http://olabini.com/blog/2010/04/patterns-of-method-missing/</ref> 


Construct                          Language 
AUTOLOAD                           Perl
AUTOSCALAR, AUTOMETH, AUTOLOAD...  Perl6
__getattr__                        Python
method_missing                     Ruby
doesNotUnderstand                  Smalltalk
__noSuchMethod__(17)               CoffeeScript, JavaScript
unknown                            Tcl
no-applicable-method               Common Lisp
doesNotRecognizeSelector           Objective-C
TryInvokeMember(18)                C#
match [name, args] { ... }         E
the predicate fail                 Prolog
forward                            Io

Table Reference<ref>http://stackoverflow.com/questions/2865865/are-there-equivalents-to-rubys-method-missing-in-other-languages</ref>

object.__getattr__(self,name)

This method is called when an attribute lookup has not found the attribute in the usual places i.e. it is not an instance attribute nor is it found in the class tree for self. name is the attribute name. This method should return the (computed) attribute value or raise an AttributeError exception.

class Roman(object): 

 def roman_to_int(self, roman):
   # implementation here
 end

 def __getattr__(self, name):
   return self.roman_to_int(name)
 end

>>> r = Roman() >>> r.iv 4


In SmalltalkLanguage When a receiver is asked to perform a method that is unknown to it, then a run-time complaint is issued which is #doesNotUnderstand. When a Smalltalk object is sent a message for a method it has not defined, the runtime system turns the message-send into an object and sends #doesNotUnderstand: to the original receiver with this message-send object as argument. By default the #doesNotUnderstand: method raises an exception, but the receiver can override it and implement it in a way that he sees fit.

  • Other languages

JavaScript also has a method which has an implementation similar to that of method_missing and that is "noSuchMethod". The limitation of this method is that it is only supported by Firefox/Spidermonkey.

Similarly, Perl has an AUTOLOAD method which works on subroutines & class/object methods.

Conclusion

method_missing is a very powerful feature of Ruby and as is the way with powerful things it can help you a great deal if used properly but it can make things a lot harder for you if implemented incorrectly. It is a kind of a feature which should be used sparingly, if not at all. And there are things that needs to be taken into consideration, as mentioned in the Key Points section above, if method_missing is to be implemented.

References

<references/>

Further Suggested Reading

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