在可变模板递归基例中去掉不必要的类

Question

下面的可变模板递归地创建类，并按顺序添加每个指定的成员：

class Dummy {};

template<class Base, typename... Members>
// using Composition = Base;
class Composition : public Base {};

template<class Base, typename FirstMember, typename... MoreMembers>
class Composition<Base, FirstMember, MoreMembers...>
    : public Composition<Base, MoreMembers...> {
protected:
    FirstMember member;
};

Composition<Composition<
    Composition<Dummy, int>, Composition<Dummy, int>*>, double> composition;

它可以工作，但我觉得终止步骤有点笨拙:它创建了一个继承自Base但不做其他任何事情的新类Composition<Base>。我更希望最后一步可以是template<class Base, typename... Members> using Composition = Base; -但这是不允许的，因为我们可能不会专门化别名模板。

Answer 1

您可以只获取Dummy的tid，并将1个参数构成停止条件：

template<typename... Members>
class Composition;

template<typename FirstMember, typename... MoreMembers>
class Composition<FirstMember, MoreMembers...>
    : public Composition<MoreMembers...> {
protected:
    FirstMember member;
};

template<typename FirstMember>
class Composition<FirstMember>
{
protected:
    FirstMember member;
};

Composition<Composition<
    Composition<int>, Composition<int>*>, double> composition;

[Live example]

如果，正如评论中所阐明的那样，Base是设计的重要部分，那么您应该能够通过添加更多的间接层来实现您想要的功能：

template <class Base, class... Members>
struct CompositionHelper;

template <class Base, class... Members>
using Composition = typename CompositionHelper<Base, Members...>::Type;

template <class Base>
struct CompositionHelper<Base>
{
  using Type = Base;
};

template <class Base, class FirstMember, class... MoreMembers>
struct CompositionHelper<Base, FirstMember, MoreMembers...>
{
  class Type : public Composition<Base, MoreMembers...>
  {
  protected:
    FirstMember member;
  };
};

Composition<Composition<
    Composition<Dummy, int>, Composition<Dummy, int>*>, double> composition;

[Live example]