How Does One - Without Inheritance - Override a Class Method and Call the Original from Within the New Method

How does one - without inheritance - override a class method and call the original from within the new method?

This is kinda hard to get your head around sometimes, but you need to open the "eigenclass" which is the singleton associated with a specific class object. the syntax for this is class << self do...end.

class Time
  alias :old_strftime :strftime

  def strftime
    puts "got here"
    old_strftime
  end
end

class Time
  class << self
    alias :old_now :now
    def now
      puts "got here too"
      old_now
    end
  end
end

t = Time.now
puts t.strftime

Python. Override a method without creating a derived class and call the original method

You don't use inheritance. What you do is called monky patching!

So you can do the following:

ModuleB.py

from ModuleA import FooClass

# Preserve the original function:
FooClass.originalFooMethode = FooClass.fooMethode

def _newFooMethod(self):
    if(not hasAttr(self,"varA "):
        self.varA = 0
    #Do some checks with varA

    #CALL ORIGINAL fooMethod
    self.originalFooMethode()

FooClass.fooMethod = _newFooMethod

Is it possible to call an overridden method in an instance of child class ES 2015?

It's JavaScript, so you can. There's no classes anyway, just prototypal inheritance with some syntactic sugar over it.

Which one do you want to call: Car's emitsCarbon specifically? That'll be

Car.prototype.emitsCarbon.call(car1);

Your object's class' direct ancestor class' emitsCarbon in general?

car1.__proto__.__proto__.emitsCarbon.call(car1);

(You're not supposed to directly access __proto__, but there's nothing actually stopping you besides the general good practice and guidelines).

Also, hybrids do emit carbon. Just a side note.

If I override a class method, is there a way I can call the original method (the one that was overridden)?

I present you with three icky ways to do this in +(void)load. In every case, name your method MyCategory_method or so.

class_getMethodImplementation() and class_replaceMethod(). Store the old IMP, and call it directly. You need to get the method's type encoding. Note that you can just use a normal C function too...
class_getInstanceMethod(), method_getImplementation(), method_setImplementation(). As above, but you don't need to get the method's type encoding.
class_getInstanceMethod() on both methods, and then method_exchangeImplementations(). Call MyCategory_method to get the original implementation. This is the easiest way to do it.

Sometimes, it's the only reasonably easy way to make it do what you want...

EDIT: And only do this if you know what you're doing!

How to override an inherited method from within another inherited class

That's a simultaneously fun and annoying problem to have! :) I believe that one of the missing links here is this:

The override modifier is related to virtual or abstract methods; a method with an override modifier simply provides an alternative implementation to an existing virtual or abstract method. A virtual method has a default implementation, an abstract method has no implementation at all.
The new modifier however can be applied to any method and simply allows the reuse of a name that was already taken. These methods are not related to each other at all, the only similarity being that they forcibly share the same name.

The problem with new is that the type using a new method "pretends" that the original implementation never existed. The base class however is absolutely unaware of this - new severs the link in the hierarchy, and this is what's casing your problem.

Generally speaking, you do not want to use the new modifier.

That, and you probably wanted to use var m = new Program(); instead - reasons being explained below.

Consider these two pieces of code:

LogThings a = new DoMath();
a.WriteLog("something");

and

LogThings a = new Program();
a.WriteLog("something");

At this point, the method being called is LogThings.WriteLog(). Even though we instantiate a DoMath or Program class that provides a new method, the LogThings part of the world doesn't "know" that. It instead believes to have a virtual method that doesn't happen to be overridden. As a result, this code prints:

This is the base in LogThings something

As mentioned above: new severs that link.

In the next example, the method being called is indeed DoMath.WriteLog(), because we're now simply instantiating the DoMath() class and call its LogThings method.

DoMath b = new DoMath();
b.WriteLog("something");

Not surprisingly, this code prints

this is overriding the base in DoMath
This is the base in LogThings something

Note that it does not print "this is not overriding" because we did not instantiate an instance of the Program class. Likewise, the base.LogThings() call has nothing to do with "overriding", it simply changes the focus to the LogThings type and calls whatever implementation it knows.

This is similar to the original code you used:

var m = new DoMath();

Lastly, consider this version:

DoMath c = new Program();
c.WriteLog("something");

Here, the Program class actually overrides the virtual void WriteLog method of DoMath. Consequently, this code prints

this is not overriding.

... which is now wrong, because it does.

The key to understanding this is that each class containing virtual or abstract methods has what's called a virtual function table, or vtable. This vtable is "inherited" by derived classes and allows the compiler to know which implementation of a method to call (through a so-called virtual dispatch).

You can consider an entry in the vtable to be something like a pointer to the actual implementation of a method (the one from the current class), followed by a pointer to the previous implementation.

In your example of DoMath and Program, instantiating the DoMath class would produce a vtable consisting of only

DoMath.WriteLog(string) -> null

whereas instantiating the Program class would produce an entry like this:

Program.WriteLog(string) -> DoMath.WriteLog(string) -> null

This is why ((DoMath)new Program()).WriteLog() works - even though we look at a DoMath reference, the compiler looks up the vtable for the instantiated type (Program) and can follow the chain up to the actual implementation (Program.WriteLog).

Do however note the makeshift null in there. Because the DoMath class declares the WriteLog method as new, it is considered to be a - well - new method, which is unrelated to the one from LogThings. For LogThings, the vtable world still looks somewhat like this:

LogThings.WriteLog(string) -> null

Because there is no legit override - just a different method that happens to have the same name - ((LogThings)new Program()).WriteLog() calls LogThings.WriteLog(), as that's the last implementation in the chain. The new essentially "forces in" another vtable, resulting in this somewhat split-brained setup.

Please note that this description of a vtable is drastically oversimplified; there's plenty of good material out there on that topic however.

When monkey patching an instance method, can you call the overridden method from the new implementation?

EDIT: It has been 9 years since I originally wrote this answer, and it deserves some cosmetic surgery to keep it current.

You can see the last version before the edit here.

You can’t call the overwritten method by name or keyword. That’s one of the many reasons why monkey patching should be avoided and inheritance be preferred instead, since obviously you can call the overridden method.

Avoiding Monkey Patching

Inheritance

So, if at all possible, you should prefer something like this:

class Foo
  def bar
    'Hello'
  end
end 

class ExtendedFoo < Foo
  def bar
    super + ' World'
  end
end

ExtendedFoo.new.bar # => 'Hello World'

This works, if you control creation of the Foo objects. Just change every place which creates a Foo to instead create an ExtendedFoo. This works even better if you use the Dependency Injection Design Pattern, the Factory Method Design Pattern, the Abstract Factory Design Pattern or something along those lines, because in that case, there is only place you need to change.

Delegation

If you do not control creation of the Foo objects, for example because they are created by a framework that is outside of your control (like ruby-on-rails for example), then you could use the Wrapper Design Pattern:

require 'delegate'

class Foo
  def bar
    'Hello'
  end
end 

class WrappedFoo < DelegateClass(Foo)
  def initialize(wrapped_foo)
    super
  end

  def bar
    super + ' World'
  end
end

foo = Foo.new # this is not actually in your code, it comes from somewhere else

wrapped_foo = WrappedFoo.new(foo) # this is under your control

wrapped_foo.bar # => 'Hello World'

Basically, at the boundary of the system, where the Foo object comes into your code, you wrap it into another object, and then use that object instead of the original one everywhere else in your code.

This uses the Object#DelegateClass helper method from the delegate library in the stdlib.

“Clean” Monkey Patching

`Module#prepend`: Mixin Prepending

The two methods above require changing the system to avoid monkey patching. This section shows the preferred and least invasive method of monkey patching, should changing the system not be an option.

Module#prepend was added to support more or less exactly this use case. Module#prepend does the same thing as Module#include, except it mixes in the mixin directly below the class:

class Foo
  def bar
    'Hello'
  end
end 

module FooExtensions
  def bar
    super + ' World'
  end
end

class Foo
  prepend FooExtensions
end

Foo.new.bar # => 'Hello World'

Note: I also wrote a little bit about Module#prepend in this question: Ruby module prepend vs derivation

Mixin Inheritance (broken)

I have seen some people try (and ask about why it doesn’t work here on StackOverflow) something like this, i.e. includeing a mixin instead of prepending it:

class Foo
  def bar
    'Hello'
  end
end 

module FooExtensions
  def bar
    super + ' World'
  end
end

class Foo
  include FooExtensions
end

Unfortunately, that won’t work. It’s a good idea, because it uses inheritance, which means that you can use super. However, Module#include inserts the mixin above the class in the inheritance hierarchy, which means that FooExtensions#bar will never be called (and if it were called, the super would not actually refer to Foo#bar but rather to Object#bar which doesn’t exist), since Foo#bar will always be found first.

Method Wrapping

The big question is: how can we hold on to the bar method, without actually keeping around an actual method? The answer lies, as it does so often, in functional programming. We get a hold of the method as an actual object, and we use a closure (i.e. a block) to make sure that we and only we hold on to that object:

class Foo
  def bar
    'Hello'
  end
end 

class Foo
  old_bar = instance_method(:bar)

  define_method(:bar) do
    old_bar.bind(self).() + ' World'
  end
end

Foo.new.bar # => 'Hello World'

This is very clean: since old_bar is just a local variable, it will go out of scope at the end of the class body, and it is impossible to access it from anywhere, even using reflection! And since Module#define_method takes a block, and blocks close over their surrounding lexical environment (which is why we are using define_method instead of def here), it (and only it) will still have access to old_bar, even after it has gone out of scope.

Short explanation:

old_bar = instance_method(:bar)

Here we are wrapping the bar method into an UnboundMethod method object and assigning it to the local variable old_bar. This means, we now have a way to hold on to bar even after it has been overwritten.

old_bar.bind(self)

This is a bit tricky. Basically, in Ruby (and in pretty much all single-dispatch based OO languages), a method is bound to a specific receiver object, called self in Ruby. In other words: a method always knows what object it was called on, it knows what its self is. But, we grabbed the method directly from a class, how does it know what its self is?

Well, it doesn’t, which is why we need to bind our UnboundMethod to an object first, which will return a Method object that we can then call. (UnboundMethods cannot be called, because they don’t know what to do without knowing their self.)

And what do we bind it to? We simply bind it to ourselves, that way it will behave exactly like the original bar would have!

Lastly, we need to call the Method that is returned from bind. In Ruby 1.9, there is some nifty new syntax for that (.()), but if you are on 1.8, you can simply use the call method; that’s what .() gets translated to anyway.

Here are a couple of other questions, where some of those concepts are explained:

How do I reference a function in Ruby?
Is Ruby’s code block same as C♯’s lambda expression?

“Dirty” Monkey Patching

`alias_method` chain

The problem we are having with our monkey patching is that when we overwrite the method, the method is gone, so we cannot call it anymore. So, let’s just make a backup copy!

class Foo
  def bar
    'Hello'
  end
end 

class Foo
  alias_method :old_bar, :bar

  def bar
    old_bar + ' World'
  end
end

Foo.new.bar # => 'Hello World'
Foo.new.old_bar # => 'Hello'

The problem with this is that we have now polluted the namespace with a superfluous old_bar method. This method will show up in our documentation, it will show up in code completion in our IDEs, it will show up during reflection. Also, it still can be called, but presumably we monkey patched it, because we didn’t like its behavior in the first place, so we might not want other people to call it.

Despite the fact that this has some undesirable properties, it has unfortunately become popularized through AciveSupport’s Module#alias_method_chain.

An aside: Refinements

In case you only need the different behavior in a few specific places and not throughout the whole system, you can use Refinements to restrict the monkey patch to a specific scope. I am going to demonstrate it here using the Module#prepend example from above:

class Foo
  def bar
    'Hello'
  end
end 

module ExtendedFoo
  module FooExtensions
    def bar
      super + ' World'
    end
  end

  refine Foo do
    prepend FooExtensions
  end
end

Foo.new.bar # => 'Hello'
# We haven’t activated our Refinement yet!

using ExtendedFoo
# Activate our Refinement

Foo.new.bar # => 'Hello World'
# There it is!

You can see a more sophisticated example of using Refinements in this question: How to enable monkey patch for specific method?

Abandoned ideas

Before the Ruby community settled on Module#prepend, there were multiple different ideas floating around that you may occasionally see referenced in older discussions. All of these are subsumed by Module#prepend.

Method Combinators

One idea was the idea of method combinators from CLOS. This is basically a very lightweight version of a subset of Aspect-Oriented Programming.

Using syntax like

class Foo
  def bar:before
    # will always run before bar, when bar is called
  end

  def bar:after
    # will always run after bar, when bar is called
    # may or may not be able to access and/or change bar’s return value
  end
end

you would be able to “hook into” the execution of the bar method.

It is however not quite clear if and how you get access to bar’s return value within bar:after. Maybe we could (ab)use the super keyword?

class Foo
  def bar
    'Hello'
  end
end 

class Foo
  def bar:after
    super + ' World'
  end
end

Replacement

The before combinator is equivalent to prepending a mixin with an overriding method that calls super at the very end of the method. Likewise, the after combinator is equivalent to prepending a mixin with an overriding method that calls super at the very beginning of the method.

You can also do stuff before and after calling super, you can call super multiple times, and both retrieve and manipulate super’s return value, making prepend more powerful than method combinators.

class Foo
  def bar:before
    # will always run before bar, when bar is called
  end
end

# is the same as

module BarBefore
  def bar
    # will always run before bar, when bar is called
    super
  end
end

class Foo
  prepend BarBefore
end

and

class Foo
  def bar:after
    # will always run after bar, when bar is called
    # may or may not be able to access and/or change bar’s return value
  end
end

# is the same as

class BarAfter
  def bar
    original_return_value = super
    # will always run after bar, when bar is called
    # has access to and can change bar’s return value
  end
end

class Foo
  prepend BarAfter
end

`old` keyword

This idea adds a new keyword similar to super, which allows you to call the overwritten method the same way super lets you call the overridden method:

class Foo
  def bar
    'Hello'
  end
end 

class Foo
  def bar
    old + ' World'
  end
end

Foo.new.bar # => 'Hello World'

The main problem with this is that it is backwards incompatible: if you have method called old, you will no longer be able to call it!

Replacement

super in an overriding method in a prepended mixin is essentially the same as old in this proposal.

`redef` keyword

Similar to above, but instead of adding a new keyword for calling the overwritten method and leaving def alone, we add a new keyword for redefining methods. This is backwards compatible, since the syntax currently is illegal anyway:

class Foo
  def bar
    'Hello'
  end
end 

class Foo
  redef bar
    old + ' World'
  end
end

Foo.new.bar # => 'Hello World'

Instead of adding two new keywords, we could also redefine the meaning of super inside redef:

class Foo
  def bar
    'Hello'
  end
end 

class Foo
  redef bar
    super + ' World'
  end
end

Foo.new.bar # => 'Hello World'

Replacement

redefining a method is equivalent to overriding the method in a prepended mixin. super in the overriding method behaves like super or old in this proposal.

C# Override virtual function without having to implement another class

if you have control over MyClass, you can let the desired method call a delegate which can be replaced for every single object at runtime...

class MyClass
{
    public void Func<SomeParameterType,SomeReturnType> myDelegate {get;set;}
    public SomeReturnType myFunction(SomeParameterType parameter)
    {
        if(myDelegate==null)
            throw new Exception();
        return myDelegate(parameter);
    }
}

...

MyClass obj = new MyClass();
SomeParameterType p = new SomeParameterType();
obj.myDelegate = (x)=>new SomeReturnType(x);
SomeReturnType result = obj.myFunction(p);

How Does One - Without Inheritance - Override a Class Method and Call the Original from Within the New Method