C++ variadic template delegation cycle error

Question

I want to write a helper structure testing a static condition on classes. If the condition is true, a object should be allocated in heap and a pointer to the object should be emplacd back to a std::vector.

Those objects look like:

class BASE {
   public:
      virtual void func() = 0;
};

class A : public BASE {
   public:
      const static int I = 0;

      void func() {
         std::cout << "CLASS A" << endl;
      }
};

class B : public BASE {
   public:
      const static int I = 1;

      void func() {
         std::cout << "CLASS B" << endl;
      }
};

The check structure:

template<class... R>
struct cond {};

template<class T, class... R>
struct cond<T, R...> : cond<R...> {
   cond( vector<BASE *> &_b ) : cond( _b ) {
      if( T::I == 1 )
         _b.emplace_back( new T() );
   }
};

And somewhere in the main function:

std::vector<BASE *> b;
cond<A, B> t(b);
for( auto *x : b ) {
   x->func();
}

In theory the constructor in the cond structure should call the constructor of its parent, but C++11 also introduced a feature to call constructors in constructors (delegation). So the compiler seams to think that I want to call a constructor in the same class, resulting in this error:

./main.cpp:83:34: error: constructor for 'cond' creates a delegation cycle [-Wdelegating-ctor-cycles]

Simply moving the vector to a global scope and removing the constructor arguments works but I would prefere an other solution.

Is it possible to tell the compiler somehow to interprete cond( _b ) right?

Answer 1

Just make it explicit which part of class are you using, by giving complete type:

template<class... R>
struct cond {};

template<class T, class... R>
struct cond<T, R...> : cond<R...> {
   cond( vector<BASE *> &_b ) : cond<R...>( _b ) {
      if( T::I == 1 )
         _b.emplace_back( new T() );
   }
};

After the : in constructor give full type, exactly as it is provided in the inheritance list of the class - cond<R...>

EDIT: As for the error, that no constructor is found, notice that it is true. This class:

template<class... R>
struct cond {};

Doesn't have one, so you should add something like this and it should work:

template<class... R>
struct cond
{
    template<typename...T>
    cond(T...)
    {

    }
};