C++怎么实现一个访问者模式(Visitor)_C++设计模式与访问者模式实现

#include <iostream>
#include <vector>
#include <memory>

// 前向声明
class TextElement;
class ImageElement;

// 访问者接口
class Visitor {
public:
    virtual ~Visitor() = default;
    virtual void visit(TextElement& element) = 0;
    virtual void visit(ImageElement& element) = 0;
};

// 元素基类
class Element {
public:
    virtual ~Element() = default;
    virtual void accept(Visitor& visitor) = 0;
};

// 具体元素：文本
class TextElement : public Element {
public:
    void accept(Visitor& visitor) override {
        visitor.visit(*this);
    }

    std::string getText() const {
        return "This is a text block.";
    }
};

// 具体元素：图片
class ImageElement : public Element {
public:
    void accept(Visitor& visitor) override {
        visitor.visit(*this);
    }

    std::string getImageInfo() const {
        return "Image: photo.jpg, size 1920x1080";
    }
};

// 具体访问者：打印访问者
class PrintVisitor : public Visitor {
public:
    void visit(TextElement& element) override {
        std::cout << "[Print] " << element.getText() << "\n";
    }

    void visit(ImageElement& element) override {
        std::cout << "[Print] " << element.getImageInfo() << "\n";
    }
};

// 具体访问者：导出为 HTML
class HtmlExportVisitor : public Visitor {
public:
    void visit(TextElement& element) override {
        std::cout << "<p>" << element.getText() << "</p>\n";
    }

    void visit(ImageElement& element) override {
        std::cout << "<img src='" << element.getImageInfo().substr(7, 10) 
                  << "' alt='image'/>\n"; // 简化提取文件名
    }
};

// 文档结构（对象结构）
class Document {
private:
    std::vector<std::unique_ptr<Element>> elements;

public:
    void addElement(std::unique_ptr<Element> element) {
        elements.push_back(std::move(element));
    }

    void accept(Visitor& visitor) {
        for (auto& e : elements) {
            e->accept(visitor);
        }
    }
};

int main() {
    Document doc;
    doc.addElement(std::make_unique<TextElement>());
    doc.addElement(std::make_unique<ImageElement>());

    PrintVisitor printVisitor;
    HtmlExportVisitor htmlVisitor;

    std::cout << "=== 打印文档 ===\n";
    doc.accept(printVisitor);

    std::cout << "\n=== 导出为 HTML ===\n";
    doc.accept(htmlVisitor);

    return 0;
}

=== 打印文档 ===
[Print] This is a text block.
[Print] Image: photo.jpg, size 1920x1080
<p>=== 导出为 HTML ===</p><p>This is a text block.</p>@@##@@