C++ 11模板,参数包的别名

Question

在个人项目中我有这样的事情:

template <typename T>
class Base {
    //This class is abstract.
} ;

template <typename T>
class DerivedA : public Base<T> {
    //...
} ;

template <typename T>
class DerivedB : Base<T> {
    //...
} ;

class Entity : public DerivedA<int>, DerivedA<char>, DerivedB<float> {
    //this one inherits indirectly from Base<int>, Base<char> & Base<float>
};

"Base"类是一种适配器,它让我将"Entity"视为int,char,float或者我想要的任何东西.DerivedA和DerivedB有不同的转换方式.然后我有一个类,让我存储我的实体的不同视图,如下所示:

template <typename... Args>
class BaseManager {
  public:
    void store(Args*... args){
        //... do things
    }
};

我有很多不同的"实体"类,它们有不同的"基础"集合.我希望能够在以下别名中存储类型列表:

class EntityExtra : public DerivedA<int>, DerivedA<char>, DerivedB<float>{
  public:
    using desiredBases = Base<int>, Base<char>, Base<float>; /* here is the problem */
};

所以我可以这样使用它:

EntityExtra ee;
BaseManager<Base<int>, Base<char>, Base<float> > bm;  // <- I can use it this way
BaseManager<EntityExtra::desiredBases> bm;            // <- I want to use it this way
bm.store(&ee,&ee,&ee);  // The first ee will be converted to a Base<int>, the second to Base<char>  and so on

有没有办法为任意类型的列表创建别名,然后在模板参数包中使用它？

Answer 1

你可能想要这个:

template <typename ...P> struct parameter_pack
{
    template <template <typename...> typename T> using apply = T<P...>;
};

// Example usage:

struct A {};
struct B {};
struct C {};

template <typename...> struct S {};

using my_pack = parameter_pack<A, B, C>;

my_pack::apply<S> var; // Equivalent to `S<A, B, C> var;`.

在你的情况下,它可以像这样使用:

class EntityExtra : public DerivedA<int>, DerivedA<char>, DerivedB<float>{
  public:
    using desiredBases = parameter_pack<Base<int>, Base<char>, Base<float>>;
};

// ...

EntityExtra::desiredBases::apply<BaseManager> bm;
// Creates `BaseManager<Base<int>, Base<char>, Base<float>> bm;`.