如何在工厂设计中支持多个构造签名?
How to support multiple construction signatures in a factory design?
我正在使用以下(简化的)工厂设计来创建某些继承层次结构的对象,应该没有什么特别的:
// class to create
class Class
{
public:
Class(Type type, Foo foo);
};
// Simple creator class.
// Used in practice to do some runtime checks about whether or not construction is allowed.
class Creator
{
public:
Class* create( Type type, Foo foo ) const
{
return new Class( type, foo );
}
};
class Factory
{
public:
Factory
{
// fill object creator map on construction
_map[ "name" ] = new Creator<Class>;
}
Class* create( const std::string& name, Type type, Foo foo )
{
// fowards to map entry
return _map[name]->create( type, foo );
}
private:
std::map<std::string, Creator*> _map;
}
// client code
int main()
{
Factory f;
factory.create(name, type, foo);
}
现在我 运行 遇到了问题,一旦我想创建具有不同构造函数签名的子class es,因为工厂在整个继承层次结构上强加了一个固定的签名。 IE。对于以下 class 我无法通过工厂构造指定新的第三个参数,而无需再次将此扩展签名强加在我的层次结构的所有其他 class 上。
class ExtClass : public Class
{
public:
Class(Type type, Foo foo, NewMember nm)
: Class(type, foo),
_nm(nm)
private:
NewMember _nm;
};
有没有办法在不进行主要更改的情况下使它适用于我当前的设计?我正在考虑使用模板或绑定对象来使不同的参数调用成为可能。
或者在这种情况下,您会建议采用与工厂设计不同的解决方案吗?
对不同的命名约定表示歉意,但这是我目前使用的 C++14 解决方案。两个主要缺点是
- 当调用
CreateObject
时,作为传递值的类型
参数必须与注册的类型相同。你不能通过
a float
并调用在 double
中注册的构造函数
签名。
- 由于
boost::bind
中的实施细节,
参数必须是 const &
.
因为我想使用 boost::factory
的设计限制是 class 的对象必须包含在 boost::function
中(以消除函数签名的歧义)。
它确实有效,但肯定可以通过更多的元编程智慧加以改进:
#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));
}
这个答案与我的第一个解决方案完全不同,它包括您可能认为 "principal changes" 我已将其作为单独答案的内容:
在我看来,它优于我之前的解决方案,但这取决于您的具体要求。这里的特点是:
- 创建者 ID 是唯一的。
CreateObject
支持参数隐式转换
存在构造函数必须采用 const&
参数的相同限制。这可能无关紧要,但此解决方案只需要 C++11。当然,使用新的 C++17 元组功能会简单一些。
#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 <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));
}
我正在使用以下(简化的)工厂设计来创建某些继承层次结构的对象,应该没有什么特别的:
// class to create
class Class
{
public:
Class(Type type, Foo foo);
};
// Simple creator class.
// Used in practice to do some runtime checks about whether or not construction is allowed.
class Creator
{
public:
Class* create( Type type, Foo foo ) const
{
return new Class( type, foo );
}
};
class Factory
{
public:
Factory
{
// fill object creator map on construction
_map[ "name" ] = new Creator<Class>;
}
Class* create( const std::string& name, Type type, Foo foo )
{
// fowards to map entry
return _map[name]->create( type, foo );
}
private:
std::map<std::string, Creator*> _map;
}
// client code
int main()
{
Factory f;
factory.create(name, type, foo);
}
现在我 运行 遇到了问题,一旦我想创建具有不同构造函数签名的子class es,因为工厂在整个继承层次结构上强加了一个固定的签名。 IE。对于以下 class 我无法通过工厂构造指定新的第三个参数,而无需再次将此扩展签名强加在我的层次结构的所有其他 class 上。
class ExtClass : public Class
{
public:
Class(Type type, Foo foo, NewMember nm)
: Class(type, foo),
_nm(nm)
private:
NewMember _nm;
};
有没有办法在不进行主要更改的情况下使它适用于我当前的设计?我正在考虑使用模板或绑定对象来使不同的参数调用成为可能。 或者在这种情况下,您会建议采用与工厂设计不同的解决方案吗?
对不同的命名约定表示歉意,但这是我目前使用的 C++14 解决方案。两个主要缺点是
- 当调用
CreateObject
时,作为传递值的类型 参数必须与注册的类型相同。你不能通过 afloat
并调用在double
中注册的构造函数 签名。 - 由于
boost::bind
中的实施细节, 参数必须是const &
.
因为我想使用 boost::factory
的设计限制是 class 的对象必须包含在 boost::function
中(以消除函数签名的歧义)。
它确实有效,但肯定可以通过更多的元编程智慧加以改进:
#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));
}
这个答案与我的第一个解决方案完全不同,它包括您可能认为 "principal changes" 我已将其作为单独答案的内容:
在我看来,它优于我之前的解决方案,但这取决于您的具体要求。这里的特点是:
- 创建者 ID 是唯一的。
CreateObject
支持参数隐式转换
存在构造函数必须采用 const&
参数的相同限制。这可能无关紧要,但此解决方案只需要 C++11。当然,使用新的 C++17 元组功能会简单一些。
#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 <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));
}