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manau's questions

Martin Hope
manau
Asked: 2025-04-09 21:48:28 +0800 CST
  • 7

我有以下 CRTP 类，我想用它们来实现访问者模式

template <class Derived>
class Visitor
{
public:
    Visitor() {
        static_assert(!std::is_same<Visitor, Derived>::value,
                      "Visitor class cannot be instantiated directly. Use a derived class.");
    }
    virtual ~Visitor() = default;
    virtual void visit(Derived&) = 0;
};

template <class Derived>
class Visitable
{
friend void Visitor<Derived>::visit(Derived&);

public:
    Visitable() {
        static_assert(!std::is_same<Visitable, Derived>::value,
                      "Visitable class cannot be instantiated directly. Use a derived class.");
    }

    template <class T>
    void accept(T& visitor)
    {
        visitor.visit(static_cast<Derived &>(*this));
    }
};

然后我定义一个可访问的类

class Data : public Visitable<Data>
{
    private:
        std::string foo;
};

以及班级的访客

class Printer : public Visitor<Data>
{
    void visit(Data& data)
    {
        std::cout << data.foo << std::endl;
    };
};

但是编译器告诉我“foo 是‘Data’的私有成员”......为什么我的朋友规范不能按预期工作？

我也尝试将“朋友”移动到派生类，但并没有解决问题。

class Data : public Visitable<Data>
{
    friend void Visitor<Data>::visit(Data&); // instead of in Visitable
    private:
        std::string foo;
};
c++