dynamic_cast具有「已知」繼承的模板類

Question

我有一個從模板定義類型繼承的類型。模板定義的類型保證有一個給定的基類。我想要做的是能夠dynamic_cast或以其他方式查找與匹配派生類型的容器中的類型，而不考慮模板參數。

// A bunch of classes exist which inherit from Base already. 
class Base{}; 
class Base2 : public Base {}; 
class BaseN : public Base {}; 

// Some new classes can inherit from any Base-derived class, 
// but also have special attributes (i.e. function "f"). 
template<typename T = Base> 
class Derived : public T 
{ 
    static_assert(std::is_base_of<Base, T>::value, 
     "Class must inherit from a type derived from Base.") 

    public: 
     void f(); 
}; 

// 
// Now process a collection of Base pointers. 
// 

std::vector<Base*> objects; 

// The vector may contain classes that are not "Derived". 
// I only care about the ones that are. 
// I want the cast here to return non-null for Derived<Base>, 
// Derived<Base2>, Derived<BaseN>, but null for Base, Base2, etc. 

// This will be Null, Good. 
objects.push_back(new Base) 
auto dcast0 = dynamic_cast<Derived<Base>*>(objects[0]); 

// This will be Non Null, Good. 
objects.push_back(new Derived<Base>); 
auto dcast1 = dynamic_cast<Derived<Base>*>(objects[1]); 

// This will be Null, BAD! HELP! 
objects.push_back(new Derived<Base2>); 
auto dcast2 = dynamic_cast<Derived<Base>*>(objects[2]); 

Answer 1

如建議在評論Creris'，你需要一個非模板基類是通用於所有的Derived<>模板類。另外，對Base本身的繼承應該是虛擬的，這樣當Derived<>被實例化時，只有一個Base實例。

struct Base { virtual ~Base() {} }; 
struct Base2 : virtual Base {}; 

struct DerivedBase : virtual Base {}; 

template <typename BASE = Base> 
struct Derived : DerivedBase, BASE {}; 

    Base *b = new Derived<Base2>; 
    assert(dynamic_cast<DerivedBase *>(b));