Get shadowed members of variadic template with recursive inheritance by type

Question

I have something like the following:

template <typename T>
class A {
public:
    typedef T Type;
};

template <typename...> class B;

template <typename TFirst, typename... TRest>
class B<TFirst, TRest...> : public B<TRest...> {
public:
    typedef typename TFirst::Type Type;

    TFirst a;

    virtual void func(TFirst a, Type t) = 0;
};

template <>
class B<> {};

struct X {};
struct Y {};

class ASub1 : public A<X> {};
class ASub2 : public A<Y> {};

class BSub : public B<ASub1, ASub2> {
public:
    // Implement all pure virtual methods
    // void func(ASub1 a1, Type t) override {}
    // void func(ASub2 a2, Type t) override {}
};

Now I have two questions:

1. How would I be able to implement all pure virtual methods in BSub ? I somehow need to access the template arguments (types) of all ASubX .

2. Is there a way to access all the members a by passing the ASubX ? I mean something like get<ASub1>(b_sub) if b_sub were an instance of BSub .

I would prefer a solution with C++14.

Answer 1

For (1), you can write

class BSub : public B<ASub1, ASub2> {
public:
    void func (ASub1, typename ASub1::Type) override {}
    void func (ASub2, typename ASub2::Type) override {}
};

or, remebering that ASub1::Type is X and ASub2::Type is Y ,

class BSub : public B<ASub1, ASub2> {
public:
    void func (ASub1, X) override {}
    void func (ASub2, Y) override {}
};

For (2), as suggested by WF, you can use

b_sub.B<ASub1, ASub2>::a

to access the ASub1 component and

b_sub.B<ASub2>::a

to access the ASub2 one.

The following is a full working (simplified) example

#include <iostream>
#include <type_traits>

template <typename T>
struct A
 { using Type = T; };

template <typename...>
struct B;

template <typename TFirst, typename ... TRest>
struct B<TFirst, TRest...> : public B<TRest...>
 {
    using Type = typename TFirst::Type;

    TFirst a;

    virtual void func (TFirst a, Type t) = 0;
 };

template <>
struct B<> {};

struct X {};
struct Y {};

struct ASub1 : public A<X> {};
struct ASub2 : public A<Y> {};

struct BSub : public B<ASub1, ASub2>
 {
   void func (ASub1, X) override { std::cout << 1 << std::endl; }
   void func (ASub2, Y) override { std::cout << 2 << std::endl; }
 };

int main()
 {
   BSub bs;

   bs.func(ASub1{}, X{});
   bs.func(ASub2{}, Y{});

   using T1 = decltype(bs.B<ASub1, ASub2>::a);
   using T2 = decltype(bs.B<ASub2>::a);

   static_assert(std::is_same<T1, ASub1>{}, "!");
   static_assert(std::is_same<T2, ASub2>{}, "!");
 }

Answer 2

Inspired by an implementation of std::get for std::tuple I came up with following solution.

Thanks for your help.

#include <cstdio>

template <typename T>
class A {
public:
    using Type = T;
};

template <typename...> class B;

template <typename TFirst, typename... TRest>
class B<TFirst, TRest...> : public B<TRest...> {
public:
    using AType = typename TFirst::Type;
    using Type = B<TFirst, TRest...>;

    TFirst a;

    virtual void func(TFirst a, AType t) {}
};

template <>
class B<> {};

struct X {};
struct Y {};

class ASub1 : public A<X> { public: int value = 10; };
class ASub2 : public A<Y> { public: int value = 20; };

class BSub : public B<ASub1, ASub2> {
public:
    // Implement all pure virtual methods
    void func(ASub1 a1, typename ASub1::Type t) {}
    void func(ASub2 a2, typename ASub2::Type t) {}
};

template <typename TA, typename TB> class BElement;

template <typename TA, typename TAFirst, typename... TARest>
class BElement<TA, B<TAFirst, TARest...>> : public BElement<TA, B<TARest...>> {
public:
};

template <typename TA, typename... TARest>
class BElement<TA, B<TA, TARest...>> {
public:
    using AType = TA;
    using BType = B<TA, TARest...>;
};

template <typename TA, typename... TAs>
TA& get(B<TAs...> & b) {
    using BType = typename BElement<TA, B<TAs...>>::BType;
    return static_cast<BType &>(b).a;
}

template <typename TA, typename TB>
TA& get(TB b) {
    return get<TA>(static_cast<typename TB::Type &>(b));
}

int main() {
  BSub bsub;
  printf("%d\n", get<ASub1>(bsub).value);
  printf("%d\n", get<ASub2>(bsub).value);

  return 0;
}