具有多个构造函数签名的 C++ 通用工厂？

Question

有没有人结合过 Andrei Alexandrescu 的经典通用工厂（Chapter 8 in Modern C++ Design) with the 'multifunction' capabilities of Boost.TypeErasure 第 208 页？也就是说，可以灵活地拥有多个创建者函数签名，这些签名在参数的数量和类型方面有所不同（但仍然具有相同的 return 类型并且在编译时已知）。

也就是说，如何组合这个稍微简化的泛型Factory：

#include <map>
#include <utility>
#include <stdexcept>

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

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

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

private:
    std::map<IdentifierType, ProductCreator> associations_;
};

使用此（不完整）函数类型擦除 'pattern':

#include <boost/type_erasure/any.hpp>
#include <boost/type_erasure/builtin.hpp>
#include <boost/type_erasure/callable.hpp>
#include <boost/mpl/vector.hpp>
#include <boost/variant.hpp>    

template<class... Sig>
using multifunction = any< mpl::vector< copy_constructible<>, typeid_<>, relaxed, callable<Sig>... > >;
using variant_type = boost::make_recursive_variant< void, double, ... >::type;
using function_type = multifunction<AbstractProduct(void), AbstractProduct(double), AbstractProduct(double, double)>;

class variant_handler
{
public:
    void handle(const variant_type& arg) {
        boost::apply_visitor(impl, arg);
    }
    void set_handler(function_type f) {
        impl.f = f;
    }
private:
    struct dispatcher : boost::static_visitor<void>
    {
        template<class T>
        void operator()(const T& t) { f(t); }
        // For a vector, we recursively operate on the elements
        void operator()(const vector_type& v)
        {
            boost::for_each(v, boost::apply_visitor(*this));
        }
        function_type f;
    };
    dispatcher impl;
};

以便最终可以像这样使用它：

Factory<Arity*, int, ???> factory;
factory.Register(0, boost::bind( boost::factory<Nullary *>() ));
factory.Register(1, boost::bind( boost::factory<Unary *>(), _1 ));
auto x = factory.CreateObject(0);
auto y = factory.CreateObject(1, 0.5);

我还没有在野外找到一个现有的实现，我目前正在尝试自己制作它。我的第一次尝试犯了一个错误，试图将 boost::bind() 的结果存储在 function_type 中，这导致了与 this SO 问题相同的错误。我怀疑答案需要将 ProductCreator 模板参数移动到 Register 函数并在那里做一些事情。

所以我想我最终是在寻找通用多功能工厂的完整、有效的实现，它可能已经存在，但我只是忽略了它。但是，我们将不胜感激任何将其整合在一起的帮助。

我更喜欢 C++11 解决方案，但显然 C++14 比 none 等更好

在此先感谢您的帮助！

Answer 1

好的，我有一个不使用 Boost.TypeErasure 的略显丑陋的解决方案，它是 C++14，但它确实提供了基本相同的功能。它是 multi-tiered，所以 ID 编号是 per-factory（但你也可以唯一编号）。我会尽快写更多，但我现在真的要睡觉了...

#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));
}

Answer 2

Hallelujah，我找到了一个使用 Boost.Variant 但没有类型擦除的解决方案。我认为这比我之前的回答要好得多，因为：

创建者 ID 是唯一的。
CreateObject支持参数到构造函数的隐式转换。

存在构造函数必须采用 const& 参数的相同限制。

我已经稍微简化了整体设计以专注于基本行为。缺少的是错误处理和可配置关联容器类型的策略，这应该是额外的 class 模板参数。我还留下了一些最小的调试代码，以便您在测试时亲眼看到它是否有效。

#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));
}

具有多个构造函数签名的 C++ 通用工厂？

C++ generic factory with multiple constructor signatures?

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