How to implement the factory method pattern in C++ correctly

There's this one thing in C++ which has been making me feel uncomfortable for quite a long time, because I honestly don't know how to do it, even though it sounds simple:

How do I implement Factory Method in C++ correctly?

Goal: to make it possible to allow the client to instantiate some object using factory methods instead of the object's constructors, without unacceptable consequences and a performance hit.

By "Factory method pattern", I mean both static factory methods inside an object or methods defined in another class, or global functions. Just generally "the concept of redirecting the normal way of instantiation of class X to anywhere else than the constructor".

Let me skim through some possible answers which I have thought of.

0) Don't make factories, make constructors.

This sounds nice (and indeed often the best solution), but is not a general remedy. First of all, there are cases when object construction is a task complex enough to justify its extraction to another class. But even putting that fact aside, even for simple objects using just constructors often won't do.

The simplest example I know is a 2-D Vector class. So simple, yet tricky. I want to be able to construct it both from both Cartesian and polar coordinates. Obviously, I cannot do:

struct Vec2 {
    Vec2(float x, float y);
    Vec2(float angle, float magnitude); // not a valid overload!
    // ...
};

My natural way of thinking is then:

struct Vec2 {
    static Vec2 fromLinear(float x, float y);
    static Vec2 fromPolar(float angle, float magnitude);
    // ...
};

Which, instead of constructors, leads me to usage of static factory methods... which essentially means that I'm implementing the factory pattern, in some way ("the class becomes its own factory"). This looks nice (and would suit this particular case), but fails in some cases, which I'm going to describe in point 2. Do read on.

another case: trying to overload by two opaque typedefs of some API (such as GU

First of all, there are cases when
object construction is a task complex
enough to justify its extraction to
another class.

I believe this point is incorrect. The complexity doesn't really matter. The relevance is what does. If an object can be constructed in one step (not like in the builder pattern), the constructor is the right place to do it. If you really need another class to perform the job, then it should be a helper class that is used from the constructor anyway.

Vec2(float x, float y);
Vec2(float angle, float magnitude); // not a valid overload!

There is an easy workaround for this:

struct Cartesian {
  inline Cartesian(float x, float y): x(x), y(y) {}
  float x, y;
};
struct Polar {
  inline Polar(float angle, float magnitude): angle(angle), magnitude(magnitude) {}
  float angle, magnitude;
};
Vec2(const Cartesian &cartesian);
Vec2(const Polar &polar);

The only disadvantage is that it looks a bit verbose:

Vec2 v2(Vec2::Cartesian(3.0f, 4.0f));

But the good thing is that you can immediately see what coordinate type you're using, and at the same time you don't have to worry about copying. If you want copying, and it's expensive (as proven by profiling, of course), you may wish to use something like Qt's shared classes to avoid copying overhead.

As for the allocation type, the main reason to use the factory pattern is usually polymorphism. Constructors can't be virtual, and even if they could, it wouldn't make much sense. When using static or stack allocation, you can't create objects in a polymorphic way because the compiler needs to know the exact size. So it works only with pointers and references. And returning a reference from a factory doesn't work too, because while an object technically can be deleted by reference, it could be rather confusing and bug-prone, see Is the practice of returning a C++ reference variable, evil? for example. So pointers are the only thing that's left, and that includes smart pointers too. In other words, factories are most useful when used with dynamic allocation, so you can do things like this:

class Abstract {
  public:
    virtual void do() = 0;
};

class Factory {
  public:
    Abstract *create();
};

Factory f;
Abstract *a = f.create();
a->do();

In other cases, factories just help to solve minor problems like those with overloads you have mentioned. It would be nice if it was possible to use them in a uniform way, but it doesn't hurt much that it is probably impossible.

Stack Overflow Q#5120768, score: 131