说我有以下几种类型:
template
<
class T,
template <class> class Storage
>
struct AbstractFactoryUnit
{
virtual ~AbstractFactoryUnit() {}
virtual typename Storage< T >::StoredType doCreate(Storage< T >) = 0;
};
和
template
<
class TypeSequence,
template <class> class ProductStorage,
template <class, template <class> class> class Unit = AbstractFactoryUnit
>
struct AbstractFactory
: boost::mpl::inherit_linearly
<
TypeSequence,
boost::mpl::inherit
<
boost::mpl::_1,
Unit< boost::mpl::_2, ProductStorage >
>
>::type
{
typedef TypeSequence Products;
template <class T>
auto create() -> typename ProductStorage< T >::StoredType
{
Unit< T, ProductStorage >& unit = *this;
unit.doCreate(ProductStorage< T >());
}
};
现在我想实现le
AbstractFactory
...一些大声笑类型:
struct Foo {};
struct Bar {};
struct Baz {};
大声笑的存储:
template <class T>
struct RawPointerStorage
{
typedef T* StoredType;
};
最后实现:
struct FooBarBaz
: AbstractFactory< boost::mpl::set< Foo, Bar, Baz >, RawPointerStorage >
{
A* doCreate(RawPointerStorage< Foo >) override
{
return new A;
}
B* doCreate(RawPointerStorage< Bar >) override
{
return new B;
}
C* doCreate(RawPointerStorage< Baz >) override
{
return new C;
}
};
不幸的是,编译器提示:
1>C:\Libs\boost\boost_1_51_0\boost/mpl/aux_/preprocessed/plain/inherit.hpp(20): error C2500: 'boost::mpl::inherit2<T1,T2>' : 'AbstractFactoryUnit<T,ProductStorage>' is already a direct base class
1> with
1> [
1> T1=AbstractFactoryUnit<boost::mpl::_2,RawPointerStorage>,
1> T2=AbstractFactoryUnit<boost::mpl::_2,RawPointerStorage>
1> ]
1> and
1> [
1> T=boost::mpl::_2,
1> ProductStorage=RawPointerStorage
1> ]
我有点困惑,因为当
AbstractFactoryUnit
仅接受一个模板参数时,它的编译就很好。我的猜测是编译器无法“解析”第二个占位符,但我应该承认我不知道为什么-因为我不太清楚boost如何在占位符上调用apply
。我将VS2012与vc100或vc110一起使用。
任何想法?
(是的,我正在使用现代C++设计中描述的
AbstractFactory
)编辑:我最终决定提供我的整个
AbstractFactory
代码,而我的问题和答案都没有掩饰。 最佳答案
我不知道为什么-在这种情况下-无法“扩展”第二个占位符,但是我发现包装表达式boost::mpl::inherit
解决了我的问题。
简而言之,这是这里的AbstractFactory
:
我们将实现封装在一个命名空间Impl
中:
namespace Impl
{
template
<
class TypeSequence,
template <class> class ProductStorage,
template <class, template <class> class> class Unit
>
struct AbstractFactory
{
private:
template <class T, class U>
struct Inherit : boost::mpl::inherit< T, Unit< U, ProductStorage > >
{};
public:
typedef typename boost::mpl::inherit_linearly
<
TypeSequence,
// the trick is on the following line
Inherit< boost::mpl::_1, boost::mpl::_2 >
>
::type Type;
};
} // namespace Impl
我们从中得出这样的结果:
template
<
class TypeSequence,
template <class> class ProductStorage = RawPointerStorage,
template <class, template <class> class> class Unit = AbstractFactoryUnit
>
struct AbstractFactory
: Impl::AbstractFactory< TypeSequence, ProductStorage, Unit >::Type
{
typedef TypeSequence Products;
template <class T>
auto create() -> typename ProductStorage< T >::StoredType
{
Unit< T, ProductStorage >& unit = *this;
return unit.doCreate(ProductStorage< T >());
}
};