Question

C ++ 03允许创建从模板参数继承的模板类：

// c++03 

struct NullType {};

template <class T0, class T1 = NullType, class T2 = NullType>
class Collector : public T0, public T1, public T2
{
};

template <class T0, class T1>
class Collector<T0, T1, NullType> : public T0, public T1
{
};

template <class T0>
class Collector<T0, NullType, NullType> : public T0
{
};

所以

typedef Collector<A, B, C> X;

eqeals to

class X: public A, public B, public C {};

C ++ 11允许更容易：

// variadic templates - great thing!
template <class ... Classes>
class C11_Collector :
    public Classes ...
{
};

包装收集器必须在继承之前包装模板参数：

template <template <class> class Wrap, class T0, class T1 = NullType, class T2 = NullType>
class Wrapping_Collector : public Wrap<T0>, public Wrap<T1>, public Wrap<T2>
{
};

template <template <class> class Wrap, class T0, class T1>
class Wrapping_Collector<Wrap, T0, T1, NullType> : public Wrap<T0>, public Wrap<T1>
{
};

template <template <class> class Wrap, class T0>
class Wrapping_Collector<Wrap, T0, NullType, NullType> : public Wrap<T0>
{
};

所以

typedef Wrapping_Collector<W, A, B> X;

eqeals to

class X: public W<A>, public W<B> {};

如何通过c ++ 11更轻松地实现Wrapping_Collector？

Answer 1

是否可以强制Wrap<NullType>为空类？然后你可以直接使用

template <template <typename> class Wrap, typename... Types>
class Wrapping_Collector : public Wrap<Types>... {
   //...
};

或者，如果双继承链而不是直接多继承是好的，那么可以从Wrapping_Collector<Wrap, A, B...>和Wrapping_Collector<Wrap, B...>派生Wrap<A>：

template <template <typename> class Wrap, typename... Types>
class Wrapping_Collector;

// The normal case
template <template <typename> class Wrap, typename Head, typename... Rest>
class Wrapping_Collector<Wrap, Head, Rest...>
    : public Wrapping_Collector<Wrap, Rest...>, Wrap<Head>
{
    //...
};

// Ignore on NullType
template <template <typename> class Wrap, typename... Rest>
class Wrapping_Collector<Wrap, NullType, Rest...>
    : public Wrapping_Collector<Wrap, Rest...>
{
    //...
};

// Base case
template <template <typename> class Wrap>
class Wrapping_Collector<Wrap> {};

Answer 2

我相信你会在C ++ 11中这样做：

// variadic templates - great thing!
template <template <class> class Wrap, class... Classes>
class C11_Wrapping_Collector : public Wrap<Classes>... 
{
};

如何包装“扩展的可变参数模板参数”？

2 个答案: