Question

有没有人将Andrei Alexandrescu的经典仿制药（Chapter 8中的Modern C++ Design的第208页）与Boost.TypeErasure的“多功能”功能结合起来？也就是说，具有多个创建者函数签名的灵活性，这些签名在参数的数量和类型方面有所不同（但仍然具有相同的返回类型并且在编译时是已知的）。

换句话说，如何结合这个略微简化的通用工厂：

#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'模式'：

#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;
};

所以最终可以使用它：

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);

我还没有在野外发现现有的实现，而且我目前仍在努力实现它。我的第一次尝试错误地尝试将boost::bind()的结果存储在function_type中，这导致this问题出现同样的错误。我怀疑答案需要将ProductCreator模板参数移到Register函数并在那里做一些事情。

所以我想我最终正在寻找一个通用的多功能工厂的完整，可行的实现，这可能已经存在，我只是忽略了它。但是，任何帮助将它组合在一起都会非常感激。

我更喜欢C ++ 11解决方案，但显然C ++ 14比没有更好等等。

提前感谢您对此的任何帮助！

Answer 1

好吧，我有一个稍微丑陋的解决方案，它没有使用Boost.TypeErasure，它是C ++ 14，但它确实提供了基本相同的功能。它是多层的，所以id编号是按工厂编号的（但你也可以编号唯一）。我很快就会写，但我现在真的要去睡觉了......

#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

哈利路亚，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：

创作者ID是唯一的。
CreateObject支持将参数隐式转换为构造函数。

存在构造函数必须采用const&参数的相同限制。

我简化了整体设计，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，它应该是附加的类模板参数。我还留下了一些最小的调试代码，以便您可以自己查看它在测试时是否有效。

#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 ++泛型工厂？

2 个答案: