oop - What's the difference between a method and a function?

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Top 5 Answer for oop - What's the difference between a method and a function?

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A function is a piece of code that is called by name. It can be passed data to operate on (i.e. the parameters) and can optionally return data (the return value). All data that is passed to a function is explicitly passed.

A method is a piece of code that is called by a name that is associated with an object. In most respects it is identical to a function except for two key differences:

  1. A method is implicitly passed the object on which it was called.
  2. A method is able to operate on data that is contained within the class (remembering that an object is an instance of a class - the class is the definition, the object is an instance of that data).

(this is a simplified explanation, ignoring issues of scope etc.)

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A method is on an object or is static in class.
A function is independent of any object (and outside of any class).

For Java and C#, there are only methods.
For C, there are only functions.

For C++ and Python it would depend on whether or not you're in a class.
But in basic English:

  • Function: Standalone feature or functionality.
  • Method: One way of doing something, which has different approaches or methods, but related to the same aspect (aka class).
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'method' is the object-oriented word for 'function'. That's pretty much all there is to it (ie., no real difference).

Unfortunately, I think a lot of the answers here are perpetuating or advancing the idea that there's some complex, meaningful difference.

Really - there isn't all that much to it, just different words for the same thing.

[late addition]

In fact, as Brian Neal pointed out in a comment to this question, the C++ standard never uses the term 'method' when refering to member functions. Some people may take that as an indication that C++ isn't really an object-oriented language; however, I prefer to take it as an indication that a pretty smart group of people didn't think there was a particularly strong reason to use a different term.

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In general: methods are functions that belong to a class, functions can be on any other scope of the code so you could state that all methods are functions, but not all functions are methods:

Take the following python example:

class Door:   def open(self):     print 'hello stranger'  def knock_door:   a_door = Door()   Door.open(a_door)  knock_door() 

The example given shows you a class called "Door" which has a method or action called "open", it is called a method because it was declared inside a class. There is another portion of code with "def" just below which defines a function, it is a function because it is not declared inside a class, this function calls the method we defined inside our class as you can see and finally the function is being called by itself.

As you can see you can call a function anywhere but if you want to call a method either you have to pass a new object of the same type as the class the method is declared (Class.method(object)) or you have to invoke the method inside the object (object.Method()), at least in python.

Think of methods as things only one entity can do, so if you have a Dog class it would make sense to have a bark function only inside that class and that would be a method, if you have also a Person class it could make sense to write a function "feed" for that doesn't belong to any class since both humans and dogs can be fed and you could call that a function since it does not belong to any class in particular.

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Simple way to remember:

  • Function → Free (Free means it can be anywhere, no need to be in an object or class)
  • Method → Member (A member of an object or class)

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