What functions to put inside a class

If I have a function (say messUp that does not need to access any private variables of a class (say room), should I write the function 开发者_如何学编程inside the class like room.messUp() or outside of it like messUp(room)? It seems the second version reads better to me.

There's a tradeoff involved here. Using a member function lets you:

• Override the implementation in derived classes, so that messing up a kitchen could involve trashing the cupboards even if no cupboards are available in a generic room.
• Decide that you need to access private variables later on, without having to refactor all the code that uses the function.
• Make the function part of an interface, so that a piece of code may require that its argument be mess-up-able.

Using an external function lets you:

• Make that function generic, so that you may apply it to rooms, warehouses and oil rigs equally (if they provide the member functions required for messing up).
• Keep the class signature small, so that creating mock versions for unit testing (or different implementations) becomes easier.
• Change the class implementation without having to examine the code for that function.

There's no real way to have your cake and eat it too, so you have to make choices. A common OO decision is to make everything a method (unless clearly idiotic) and sacrifice the three latter points, but that doesn't mean you should do it in all situations.

Any behaviour of a class of objects should be written as an instance method.

So room.messUp() is the OO way to do this.

Whether messUp has to access any private members of the class or not, is irrelevant, the fact that it's a behaviour of the room, suggests that it's an instance method, as would be cleanUp or paint, etc...

Ignoring which language, I think my first question is if messUp is related to any other functions. If you have a group of related functions, I would tend to stick them in a class.

If they don't access any class variables then you can make them static. This way, they can be called without needing to create an instance of the class.

Beyond that, I would look to the language. In some languages, every function must be a method of some class.

In the end, I don't think it makes a big difference. OOP is simply a way to help organize your application's data and logic. If you embrace it, then you would choose room.messUp() over messUp(room).

i base myself on "C++ Coding Standards: 101 Rules, Guidelines, And Best Practices" by Sutter and Alexandrescu, and also Bob Martin's SOLID. I agree with them on this point of course ;-).

If the message/function doesnt interract so much with your class, you should make it a standard ordinary function taking your class object as argument.

You should not polute your class with behaviours that are not intimately related to it. This is to repect the Single Responsibility Principle: Your class should remain simple, aiming at the most precise goal.

However, if you think your message/function is intimately related to your object guts, then you should include it as a member function of your class.