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

Question

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:<

Answer 1

I mostly agree with the accepted answer, but there is a C++11 option that has not been covered in existing answers:

• Return factory method results by value, and
• Provide a cheap move constructor.

Example:

struct sandwich {
  // Factory methods.
  static sandwich ham();
  static sandwich spam();
  // Move constructor.
  sandwich(sandwich &&);
  // etc.
};

Then you can construct objects on the stack:

sandwich mine{sandwich::ham()};

As subobjects of other things:

auto lunch = std::make_pair(sandwich::spam(), apple{});

Or dynamically allocated:

auto ptr = std::make_shared(sandwich::ham());

When might I use this?

If, on a public constructor, it is not possible to give meaningful initialisers for all class members without some preliminary calculation, then I might convert that constructor to a static method. The static method performs the preliminary calculations, then returns a value result via a private constructor which just does a member-wise initialisation.

I say 'might' because it depends on which approach gives the clearest code without being unnecessarily inefficient.