C 中的访客模式示例

代替

struct IShape
{
    virtual ~IShape() = default;

    virtual void print() const = 0;
    virtual double area() const = 0;
    virtual double perimeter() const = 0;
    // .. and so on
};

访客可以使用：

// The concrete shapes
struct Square;
struct Circle;

// The visitor interface
struct IShapeVisitor
{
    virtual ~IShapeVisitor() = default;
    virtual void visit(const Square&) = 0;
    virtual void visit(const Circle&) = 0;
};

// The shape interface
struct IShape
{
    virtual ~IShape() = default;

    virtual void accept(IShapeVisitor&) const = 0;
};

现在具体的形状：

struct Point {
    double x;
    double y;
};

struct Circle : IShape
{
    Circle(const Point& center, double radius) : center(center), radius(radius) {}
    
    // Each shape has to implement this method the same way
    void accept(IShapeVisitor& visitor) const override { visitor.visit(*this); }

    Point center;
    double radius;
};

struct Square : IShape
{
    Square(const Point& topLeft, double sideLength) :
         topLeft(topLeft), sideLength(sideLength)
    {}

    // Each shape has to implement this method the same way
    void accept(IShapeVisitor& visitor) const override { visitor.visit(*this); }

    Point topLeft;
    double sideLength;
};

那么访客：

struct ShapePrinter : IShapeVisitor
{
    void visit(const Square&) override { std::cout << "Square"; }
    void visit(const Circle&) override { std::cout << "Circle"; }
};

struct ShapeAreaComputer : IShapeVisitor
{
    void visit(const Square& square) override
    {
        area = square.sideLength * square.sideLength;
    }

    void visit(const Circle& circle) override
    {
         area = M_PI * circle.radius * circle.radius;
    }

    double area = 0;
};

struct ShapePerimeterComputer : IShapeVisitor
{
    void visit(const Square& square) override { perimeter = 4. * square.sideLength; }
    void visit(const Circle& circle) override { perimeter = 2. * M_PI * circle.radius; }

    double perimeter = 0.;
};

并使用它：

const Square square = {{-1., -1.}, 2.};
const Circle circle{{0., 0.}, 1.};
const IShape* shapes[2] = {&square, &circle};

ShapePrinter shapePrinter;
ShapeAreaComputer shapeAreaComputer;
ShapePerimeterComputer shapePerimeterComputer;

for (const auto* shape : shapes) {
    shape->accept(shapePrinter);
    std::cout << " has an area of ";

    // result will be stored in shapeAreaComputer.area
    shape->accept(shapeAreaComputer);

    // result will be stored in shapePerimeterComputer.perimeter
    shape->accept(shapePerimeterComputer); 

    std::cout << shapeAreaComputer.area
              << ", and a perimeter of "
              << shapePerimeterComputer.perimeter
              << std::endl;
}

预期输出：

Square has an area of 4, and a perimeter of 8
Circle has an area of 3.14159, and a perimeter of 6.28319

演示

说明 ：

• 在 void Square::accept(IShapeVisitor& visitor) const override { visitor.visit(*this); } 中，this 的静态类型是已知的，因此所选择的（在编译时）重载是 void IVisitor::visit(const Square&);。

• 对于 square.accept(visitor); 呼叫，通过 virtual 的动态调度用于知道要呼叫的 accept。

优点 ：

• 你可以通过添加新访问者向 IShape 类添加新功能（SerializeAsXml，…）。

缺点 ：

• 添加新的具体形状（Triangle，…）需要修改所有访客。

将所有功能作为 virtual 方法放在 IShape 中的替代方案具有相反的优点和缺点：添加新功能需要修改所有现有形状，但添加新形状不会影响现有类。