访问者模式在C++怎么实现 双重分派与元素遍历的结合

// 前向声明
class ConcreteElementA;
class ConcreteElementB;

// 访问者接口
class Visitor {
public:
    virtual void visit(ConcreteElementA* element) = 0;
    virtual void visit(ConcreteElementB* element) = 0;
    virtual ~Visitor() {}
};

// 元素接口
class Element {
public:
    virtual void accept(Visitor* visitor) = 0;
    virtual ~Element() {}
};

// 具体元素A
class ConcreteElementA : public Element {
public:
    void accept(Visitor* visitor) override {
        visitor->visit(this);
    }

    std::string operationA() {
        return "ConcreteElementA operation";
    }
};

// 具体元素B
class ConcreteElementB : public Element {
public:
    void accept(Visitor* visitor) override {
        visitor->visit(this);
    }

    int operationB() {
        return 42;
    }
};

// 具体访问者
class ConcreteVisitor : public Visitor {
public:
    void visit(ConcreteElementA* element) override {
        std::cout << "ConcreteVisitor visiting ConcreteElementA: " << element->operationA() << std::endl;
    }

    void visit(ConcreteElementB* element) override {
        std::cout << "ConcreteVisitor visiting ConcreteElementB: " << element->operationB() << std::endl;
    }
};

// 对象结构
class ObjectStructure {
public:
    void attach(Element* element) {
        elements_.push_back(element);
    }

    void detach(Element* element) {
        // 移除元素，简化实现
        for(auto it = elements_.begin(); it != elements_.end(); ++it) {
            if(*it == element) {
                elements_.erase(it);
                break;
            }
        }
    }

    void accept(Visitor* visitor) {
        for (Element* element : elements_) {
            element->accept(visitor); // 双重分派
        }
    }

private:
    std::vector<Element*> elements_;
};


int main() {
    ObjectStructure structure;
    ConcreteElementA* elementA = new ConcreteElementA();
    ConcreteElementB* elementB = new ConcreteElementB();

    structure.attach(elementA);
    structure.attach(elementB);

    ConcreteVisitor* visitor = new ConcreteVisitor();
    structure.accept(visitor);

    delete visitor;
    delete elementA;
    delete elementB;

    return 0;
}