C++ generic factory with multiple constructor signatures?

放肆的年华 提交于 2019-12-04 10:20:00

OK, I have a slightly ugly solution that doesn't use Boost.TypeErasure, it is C++14, but it does provide essentially the same functionality. It's multi-tiered, so the id numbering is per-factory (but you could number uniquely too). I'll write more soon, but I really have to go to sleep right now...

#include <boost/functional/factory.hpp>
#include <boost/function.hpp>
#include <boost/bind.hpp>

#include <cassert>
#include <map>
#include <tuple>
#include <type_traits>
#include <utility>


template <class AbstractProduct, typename IdentifierType, typename... ProductCreators>
class Factory
{
    using AssociativeContainers = std::tuple<std::map<IdentifierType, boost::function<ProductCreators>>...>;
public:
    template <typename Product, typename... Arguments>
    bool Register(const IdentifierType& id, boost::function<Product(Arguments...)> creator) {
        auto &foo = std::get<std::map<IdentifierType, boost::function<AbstractProduct(const Arguments&...)>>>(associations_);
        return foo.emplace(id, creator).second;
    }

    // This function left as an exercise to the reader...
    bool Unregister(const IdentifierType& id) {
        return associations_.erase(id) == 1;
    }

    template <typename... Arguments>
    AbstractProduct CreateObject(const IdentifierType& id, Arguments&& ... args) const {
        auto const &foo = std::get<std::map<IdentifierType, boost::function<AbstractProduct(const Arguments&...)>>>(associations_);
        auto const i = foo.find(id);
        if (i != foo.end()) {
            return (i->second)(std::forward<Arguments...>(args)...);
        }
        throw std::runtime_error("Creator not found.");
    }

private:
    AssociativeContainers associations_;
};


struct Arity {
    virtual ~Arity() = default;
};

struct Nullary : Arity {};

struct Unary : Arity {
    Unary() {}
    Unary(double x) : x(x) {}

    double x;
};


int main(void)
{
    Factory<Arity*, int, Arity*(), Arity*(const double&)> factory;
    factory.Register(0, boost::function<Arity*()>{boost::factory<Nullary*>()} );
    factory.Register(1, boost::function<Arity*(const double&)>{boost::bind(boost::factory<Unary*>(), _1)});
    auto x = factory.CreateObject(1, 2.0);
    assert(typeid(*x) == typeid(Unary));
    x = factory.CreateObject(0);
    assert(typeid(*x) == typeid(Nullary));
}

Hallelujah, I found a solution using Boost.Variant but no type erasure. I think this is much better than my earlier answer, as:

  • Creator id is unique.
  • CreateObject supports implicit conversion of parameters to constructor.

The same limitation that the constructors must take const& parameters exists.

I have simplified the overall design somewhat to focus on the essential behaviour. What's missing is the policy for error handling and configurable associative container type, which should be additional class template parameters. I have also left some minimal debugging code in so that you can see for yourself that it works when you test it out.

#include <boost/functional/factory.hpp>
#include <boost/function.hpp>
#include <boost/variant.hpp>

#include <map>
#include <stdexcept>
#include <tuple>
#include <type_traits>
#include <utility>
// Just for debugging.
#include <cassert>
#include <iostream>
#include <typeinfo>
#include <cxxabi.h>

// Tuple manipulation.

template <typename Signature>
struct signature_impl;

template <typename ReturnType, typename... Args>
struct signature_impl<ReturnType(Args...)>
{
    using return_type = ReturnType;
    using param_types = std::tuple<Args...>;
};

template <typename T>
using signature_t = signature_impl<T>;


template <std::size_t... Ints>
struct indices {};

template <std::size_t N, std::size_t... Ints>
struct build_indices : build_indices<N-1, N-1, Ints...> {};

template <std::size_t... Ints>
struct build_indices<0, Ints...> : indices<Ints...> {};

template <typename Tuple>
using make_tuple_indices = build_indices<std::tuple_size<typename std::remove_reference<Tuple>::type>::value>;

// The multiple-signature factory.
template <class AbstractProduct, typename IdentifierType, typename... ProductCreators>
class multifactory
{
    using functions = boost::variant<boost::function<ProductCreators>...>;

    std::map<IdentifierType, functions> associations_;

    template <typename Signature>
    struct dispatch_foo
    {
        template <typename CreateArgs, std::size_t... Indices>
        typename std::enable_if<std::is_convertible<CreateArgs, typename signature_t<Signature>::param_types>::value, AbstractProduct>::type
        static apply(boost::function<Signature> const &f, CreateArgs && t, indices<Indices...>)
        {
            return f(std::get<Indices>(std::forward<CreateArgs>(t))...);
        }

        template <typename CreateArgs, std::size_t... Indices>
        typename std::enable_if<!std::is_convertible<CreateArgs, typename signature_t<Signature>::param_types>::value, AbstractProduct>::type
        static apply(boost::function<Signature> const &, CreateArgs &&, indices<Indices...>)
        {
            return nullptr;
        }
    };

    template <typename... CreateArguments>
    struct dispatcher : boost::static_visitor<AbstractProduct>
    {
        std::tuple<CreateArguments...> args;

        dispatcher(CreateArguments const&... args) : args{std::forward_as_tuple(args...)} {}

        template <typename Signature>
        AbstractProduct operator()(boost::function<Signature> const &f) const
        {
            int status;
            std::cout << "visitor: " << abi::__cxa_demangle(typeid(Signature).name(), nullptr, 0, &status) << "\n";
            return dispatch_foo<Signature>::apply(f, args, make_tuple_indices<std::tuple<CreateArguments...>>{});
        }
    };

public:
    template <typename ProductCreator>
    bool Register(IdentifierType id, ProductCreator &&creator) {
        return associations_.emplace(id, std::forward<ProductCreator>(creator)).second;
    }

    bool Unregister(const IdentifierType& id) {
        return associations_.erase(id) == 1;
    }

    template <typename... Arguments>
    AbstractProduct CreateObject(const IdentifierType& id, Arguments const& ... args) {
        auto i = associations_.find(id);
        if (i != associations_.end()) {
            dispatcher<Arguments...> impl(args...);
            return boost::apply_visitor(impl, i->second);
        }
        throw std::runtime_error("Creator not found.");
    }
};


struct Arity {
    virtual ~Arity() = default;
};

struct Nullary : Arity {};

struct Unary : Arity {
    Unary() {} // Also has nullary ctor.
    Unary(int) {}
};


int main(void)
{
    multifactory<Arity*, int, Arity*(), Arity*(const int&)> factory;
    factory.Register(0, boost::function<Arity*()>( boost::factory<Nullary*>() ));
    factory.Register(1, boost::function<Arity*(const int&)>(boost::factory<Unary*>()) );
    auto a = factory.CreateObject(0);
    assert(a);
    assert(typeid(*a) == typeid(Nullary));
    auto b = factory.CreateObject(1, 2);
    assert(b);
    assert(typeid(*b) == typeid(Unary));
}
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