从基类到未知派生类的强制转换

Question

给定对象初始化如下:

Base* a = new Derived();
Container<Base> c(a);

,

class Base {
  ...
  protected:
    ~Base();
}
class Derived : public Base {...};
template <typename T>
class Container {
  private:
    T* object;
  public:
    Container(T* o) : object(o) {}
    void deleteObject() {
      delete object;  // Object must be casted to (unknown) derived type to call destructor.
    }
};

显然，这是非常简化的实际代码，但问题是我如何将object从其模板类型转换为其实际的派生类型(如果它们不同)，这是未知的?

我不能修改Base或Derived，甚至任何调用Container的代码，只能修改Container类本身。

Answer 1

您需要模板化构造函数并存储类型擦除的删除器。shared_ptr就是这样做的。

template <typename T>
class Container {
  private:
    T* object;
    std::function<void(T*)> deleter;
  public:
    template<typename U> Container(U* o) : object(o) {
        deleter = [](T* ptr) { delete static_cast<U*>(ptr); };
    }
    void deleteObject() {
        deleter(object);
    }
};

Answer 2

如果您能够更改创建代码，您可能会这样做:

template<class T>
void deleter(void* p){
  delete static_cast<T*>(p);
}
template<class T>
class Container{
private:
  T* obj;
  typedef void (*deleter_func)(void*);
  deleter_func obj_deleter;
public:
  Container(T* o, deleter_func df)
    : obj(o), obj_deleter(df) {}
  void deleteObject(){ obj_deleter(obj); }
};

在调用代码中:

Base* a = new Derived();
Container<Base> c(a, &deleter<Derived>);

Answer 3

提示(因为这是作业):查找关键字virtual

Answer 4

如果不能同时更改Base或Derived或使析构函数为虚函数，则可以使deleteObject成为模板函数

template <typename T>
class Container {
  private:
    T* object;
  public:
    Container(T* o) : object(o) {}

    template <typename U>
    void deleteObject() {
      U* c = static_cast<U*>(object);
      delete c;
    }
};

int main(void)
{
    Base* a = new Derived();
    Container<Base> *b = new Container<Base>(a);

    b->deleteObject<Derived>();
        return 0;
}

Answer 5

编辑:我不知道你也不能修改Container签名…

如果你可以修改你的deleteObject:

template <typename T>
class Container {
  private:
    T* object;
  public:
    Container(T* o) : object(o) {}
    template< typename PDerived >
    void deleteObject() {
      delete static_cast< PDerived* >( object ); 
    }
};
Base* a = new Derived();
Container<Base> c(a);
c.deleteObject<Derived>();

编辑:之前有人贴过同样的解决方案