#include <iostream>

using namespace std;

struct BaseA {
   int a { 100 };
   int b { 200 };
   int c { 300 };

   auto& BaseARef() { return *this; }
};

struct BaseB {
   BaseA& baseA;
   
   BaseB(BaseA& baseA) : baseA(baseA) {
       std::cout << baseA.a << "\n";
   }
};

struct BaseC {
   BaseA& baseA;

   BaseC(BaseA& baseA) : baseA(baseA) {
       std::cout << baseA.b << "\n";
   }
};

struct BaseD {
   BaseA& baseA;

   BaseD(BaseA& baseA) : baseA(baseA) {
       std::cout << baseA.c << "\n";
   }
};

template<typename ...Args>
struct Whatever : BaseA, Args... {
   Whatever() : Args(BaseARef())... {}
};

using Test = Whatever<BaseB, BaseC, BaseD>;

int main(void) {
    Test whatever;
}

100
200
300

using Test = Whatever<BaseD, BaseC, BaseB>;

300
200
100