Factory method for template classes

Question

I have an issue I'm facing where I'm trying to build a factory function that, given an ID and a type will return the correct (templated) subclass.

What this is trying to solve:

The id() values are sent across a network as soon as a connection is established, and specify to the receiver how a sequence of bytes are encoded. The receiver knows in advance the type T that it expects, but does not know how that type T is encoded on the wire until it gets this value. It also specifies how return values (of some type U, where U may or may not be the same type as T) should be marshalled when they are returned. This code is used generally, i.e. there are multiple senders/receivers that use/expect different types; the types used between a given sender/receiver pair are always fixed, however.

A basic sketch of the problem: we have a (simplified) base class that defines id()

template <typename T>
class foo
{
public:

    virtual ~foo() { }

    // Other methods

    // This must return the same value for every type T
    virtual std::uint8_t id() const noexcept = 0;
};

From there, we have some subclasses:

template <typename T>
class bar : public foo<T>
{
public:
    std::uint8_t id() const noexcept override { return 1; }
};

template <typename T>
class quux : public foo<T>
{
public:
    std::uint8_t id() const noexcept override { return 2; }
};

For the actual factory function, I need to store something that erases the type (e.g. bar, quux) so that I can store the actual creation function in a homogenous container. Effectively, I want semantics that are roughly equivalent to:

struct creation_holder
{
    // Obviously this cannot work, as we cannot have virtual template functions
    template <typename T>
    virtual foo<T>* build() const;
};

template <template <typename> class F>
struct create : public creation_holder
{
    // As above
    template <typename T>
    foo<T>* build() const override
    {
        return new F<T>();
    }
};

std::unordered_map<std::uint8_t, create*>& mapping()
{
    static std::unordered_map<std::uint8_t, create*> m;
    return m;
}

template <typename T, template <typename> class F>
bool register_foo(F<T> foo_subclass,
    typename std::enable_if<std::is_base_of<foo<T>, F<T>>::value>::type* = 0)
{
    auto& m = mapping();
    const auto id = foo_subclass.id();
    creation_holder* hold = new create<F>();
    // insert into map if it's not already present
}

template <typename T>
foo<T>* from_id(std::uint8_t id)
{
    const auto& m = mapping();
    auto it = m.find(id);
    if(it == m.end()) { return nullptr; }
    auto c = it->second;
    return c->build<T>();
}

I've played around with a number of ideas to try and get something with similar semantics, but with no luck. Is there a way to do this (I don't care if the implementation is significantly different).

Answer 1

Some utility types for passing around types and bundles of types:

template<class...Ts>
struct types_t {};
template<class...Ts>
constexpr types_t<Ts...> types{}; // C++14.  In C++11, replace types<T> with types_t<T>{}.  Then again, I don't use it.
template<class T>
struct tag_t {};
template<class T>
constexpr tag_t<T> tag{}; // C++14.  In C++11, replace tag<T> with tag_t<T>{} below

Now we write a poly ifactory.

Here is an ifactory:

template<template<class...>class Z, class T>
struct ifactory {
  virtual std::unique_ptr<Z<T>> tagged_build(tag_t<T>) const = 0;
  virtual ~ifactory() {}
};

you pass in the tag you want to build and you get out an object. Pretty simple.

We then bundle them up (this would be easier in c++17¹, but you asked for c++11):

template<template<class...>class Z, class Types>
struct poly_ifactory_impl;

The one type case:

template<template<class...>class Z, class T>
struct poly_ifactory_impl<Z,types_t<T>>:
  ifactory<Z, T>
{
  using ifactory<Z, T>::tagged_build;
};

the 2+ case:

template<template<class...>class Z, class T0, class T1, class...Ts>
struct poly_ifactory_impl<Z,types_t<T0, T1, Ts...>>:
  ifactory<Z, T0>,
  poly_ifactory_impl<Z, types_t<T1, Ts...>>
{
  using ifactory<Z, T0>::tagged_build;
  using poly_ifactory_impl<Z, types_t<T1, Ts...>>::tagged_build;
};

We import the tagged_build method down into the derived classes. This means that the most-derived poly_ifactory_impl has all of the tagged_build methods in the same overload set. We'll use this to dispatch to them.

Then we wrap it up pretty:

template<template<class...>class Z, class Types>
struct poly_ifactory:
  poly_ifactory_impl<Z, Types>
{
  template<class T>
  std::unique_ptr<Z<T>> build() const {
    return this->tagged_build(tag<T>);
  }
};

notice I'm returning a unique_ptr; returing a raw T* from a factory method is code smell.

Someone with a poly_ifactory<?> just does a ->build<T>() and ignores the tagged_ overloads (unless they want them; I leave them exposed). Each tagged_build is virtual, but build<T> is not. This is how we emulate a virtual template function.

---

This handles the interface. At the other end we don't want to have to implement each build(tag_t<T>) manually. We can solve this with the CRTP.

template<class D, class Base, template<class...>class Z, class T>
struct factory_impl : Base {
  virtual std::unique_ptr<Z<T>> tagged_build( tag_t<T> ) const override final {
    return static_cast<D const*>(this)->build_impl( tag<T> );
  }
  using Base::build;
};

template<class D, class Base, template<class...>class Z, class Types>
struct poly_factory_impl;

the 1 type case:

template<class D, class Base, template<class...>class Z, class T0>
struct poly_factory_impl<D, Base, Z, types_t<T0>> :
  factory_impl<D, Base, Z, T0>
{
  using factory_impl<D, Base, Z, T0>::tagged_build;
};

the 2+ type case:

template<class D, class Base, template<class...>class Z, class T0, class T1, class...Ts>
struct poly_factory_impl<D, Base, Z, types_t<T0, T1, Ts...>> :
  factory_impl<D, poly_factory_impl<D, Base, Z, types_t<T1, Ts...>>, Z, T0>
{
  using factory_impl<D, poly_factory_impl<D, Base, Z, types_t<T1, Ts...>>, Z, T0>::tagged_build;
};

what this does is write a series of tagged_build(tag_t<T>) overloads of the ifactory methods, and redirects them to D::build_impl(tag_t<T>), where D is a theoretical derived type.

The fancy "pass Base around" exists to avoid having to use virtual inheritance. We inherit linearly, each step implementing one tagged_build(tag<T>) overload. All of them dispatch downward non-virtually using CRTP.

Use looks like:

struct bar {};

using my_types = types_t<int, double, bar>;

template<class T>
using vec = std::vector<T>;
using my_ifactory = poly_ifactory< vec, my_types >;

struct my_factory :
  poly_factory_impl< my_factory, my_ifactory, vec, my_types >
{
  template<class T>
  std::unique_ptr< vec<T> > build_impl( tag_t<T> ) const {
    return std::make_unique< std::vector<T> >( sizeof(T) );
    // above is C++14; in C++11, use:
    // return std::unique_ptr<vec<T>>( new vec<T>(sizeof(T)) );
  }
};

and an instance of my_factory satisfies the my_ifactory interface.

In this case, we create a unique ptr to a vector of T with a number of elements equal to sizeof(T). It is just a toy.

Live example.

---

The pseudo code design.

The interface has a

template<class T> R build

function. It dispatches to

virtual R tagged_build(tag_t<T>) = 0;

methods.

The Ts in question are extracted from a types_t<Ts...> list. Only those types are supported.

On the implementation side, we create a linear inheritance of CRTP helpers. Each inherits from the last, and overrides a virtual R tagged_build(tag_t<T>).

The implementation of tagged_build uses CRTP to cast the this pointer to a more-derived class and call build_impl(tag<T>) on it. This is an example of non-runtime polymorphism.

So calls go build<T> to virtual tagged_build(tag_t<T>) to build_impl(tag<T>). Users just interact with one template; implementors just implement one template. The glue in the middle -- the virtual tagged_build -- is generated from a types_t list of types.

This is about 100 lines of "glue" or helper code, and in exchange we get effectively virtual template methods.

---

¹ in c++17 this becomes:

template<template<class...>class Z, class...Ts>
struct poly_ifactory_impl<Z,types_t<Ts...>>:
  ifactory<Z, Ts>...
{
  using ifactory<Z, Ts>::tagged_build...;
};

which is much simpler and clearer.

---

Finally, you can do something vaguely like this without a central list of types. If you know both the caller and the callee know the type you could pass a typeid or typeindex into the ifactory, pass a void* or something similar out over the virtual dispatch mechanism, and cast/check for null/do a lookup in a map to types.

The internal implementation would look similar to this one, but you wouldn't have to publish types_t as part of your formal (or binary) interface.

Externally, you would have to "just know" what types are supported. At runtime, you might get a null smart (or dumb, ick) pointer out if you pass in an unsupported type.

With a bit of care you could even do both. Expose an efficient, safe mechanism to get compile-time known types applied to a template. Also expose a "try" based interface that both uses the efficient compile-time known system (if the type matches) and falls back on the inefficient runtime checked on. You might do this for esoteric backwards binary compatibility reasons (so new software can connect over an obsolete interface to new or old API implementations and handle having an old API implementation dynamically).

But at that point, have you considered using COM?