需要一个虚拟模板成员解决方法 [英] need a virtual template member workaround

问题描述

我需要编写一个实现访问者设计模式的程序。问题是基本访问类是一个模板类。这意味着BaseVisited :: accept（）接受一个模板类作为参数，并且因为它使用'this'，我需要'this'指向对象的正确运行时实例，它也需要是虚拟的。
我想知道是否有任何方法解决这个问题。

I need to write a program implementing the visitor design pattern. The problem is that the base visitor class is a template class. This means that BaseVisited::accept() takes a template class as a parameter and since it uses 'this' and i need 'this' to point to the correct runtime instance of the object, it also needs to be virtual.
I'd like to know if there's any way around this problem.

template <typename T>
class BaseVisitor {
  public:
    BaseVisitor();
    T visit(BaseVisited *visited);
    virtual ~BaseVisitor();
}


class BaseVisited {
  BaseVisited();
  template <typename T>
    virtual void accept(BaseVisitor<T> *visitor) { visitor->visit(this); }; // problem
  virtual ~BaseVisited();
}

推荐答案

class BaseVisited;
class BaseVisitorInternal {
public:
    virtual void visit(BaseVisited*) = 0;
    virtual ~BaseVisitorInternal() {}
};
class BaseVisited {
    BaseVisited();
    virtual void accept(BaseVisitorInternal* visitor) { visitor->visit(this); }
};
template<typename T> class BaseVisitor : public BaseVisitorInternal {
    void visit(BaseVisited* visited);
};

如果你还需要BaseVisited的派生类模板，并传递他们正确的类型/重载访问，你正式死亡。

If you need BaseVisited's derived classes to be templated too AND pass their correct types/overloads to visit, you're officially dead.

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