Factory Pattern in C++ -- doing this correctly?_问答_开发者

I am relatively new to "design patterns" as they are referred to in a formal sense. I've not been a professional for very long, so I'm pretty new to this.

We've got a pure virtual interface base class. This interface class is obviously to provide the definition of what functionality its derived children are supposed to do. The current use and situation in the software dictates what type of derived child we want to use, so I recommended creating a wrapper that will communicate which type of derived child we want and return a Base pointer that points to a new derived object. This wrapper, to my understanding, is a factory.

Well, a colleague of mine created a static function in the Base class to act as the factory. This causes开发者_如何学编程 me trouble for two reasons. First, it seems to break the interface nature of the Base class. It feels wrong to me that the interface would itself need to have knowledge of the children derived from it.

Secondly, it causes more problems when I try to re-use the Base class across two different Qt projects. One project is where I am implementing the first (and probably only real implementation for this one class... though i want to use the same method for two other features that will have several different derived classes) derived class and the second is the actual application where my code will eventually be used. My colleague has created a derived class to act as a tester for the real application while I code my part. This means that I've got to add his headers and cpp files to my project, and that just seems wrong since I'm not even using his code for the project while I implement my part (but he will use mine when it is finished).

Am I correct in thinking that the factory really needs to be a wrapper around the Base class rather than the Base acting as the factory?

You do NOT want to use your interface class as the factory class. For one, if it is a true interface class, there is no implementation. Second, if the interface class does have some implementation defined (in addition to the pure virtual functions), making a static factory method now forces the base class to be recompiled every time you add a child class implementation.

The best way to implement the factory pattern is to have your interface class separate from your factory.

A very simple (and incomplete) example is below:

class MyInterface
{
public:
    virtual void MyFunc() = 0;
};

class MyImplementation : public MyInterface
{
public:
    virtual void MyFunc() {}
};

class MyFactory
{
public:
    static MyInterface* CreateImplementation(...);
};

I'd have to agree with you. Probably one of the most important principles of object oriented programming is to have a single responsibility for the scope of a piece of code (whether it's a method, class or namespace). In your case, your base class serves the purpose of defining an interface. Adding a factory method to that class, violates that principle, opening the door to a world of shi... trouble.

Yes, a static factory method in the interface (base class) requires it to have knowledge of all possible instantiations. That way, you don't get any of the flexibility the Factory Method pattern is intended to bring.

The Factory should be an independent piece of code, used by client code to create instances. You have to decide somewhere in your program what concrete instance to create. Factory Method allows you to avoid having the same decision spread out through your client code. If later you want to change the implementation (or e.g. for testing), you have just one place to edit: this may be e.g. a simple global change, through conditional compilation (usually for tests), or even via a dependency injection configuration file.

Be careful about how client code communicates what kind of implementation it wants: that's not an uncommon way of reintroducing the dependencies factories are meant to hide.

It's not uncommon to see factory member functions in a class, but it makes my eyes bleed. Often their use have been mixed up with the functionality of the named constructor idiom. Moving the creation function(s) to a separate factory class will buy you more flexibility also to swap factories during testing.

When the interface is just for hiding the implementation details and there will be only one implementation of the Base interface ever, it could be ok to couple them. In that case, the factory function is just a new name for the constructor of the actual implementation.

However, that case is rare. Except when explicit designed having only one implementation ever, you are better off to assume that multiple implementations will exist at some point in time, if only for testing (as you discovered).

So usually it is better to split the Factory part into a separate class.