C 中的訪客模式示例

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
};

訪客可以使用：

// 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;
};

現在具體的形狀：

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;
};

那麼訪客：

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.;
};

並使用它：

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;
}

預期輸出：

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

說明 ：

• 在 void Square::accept(IShapeVisitor& visitor) const override { visitor.visit(*this); } 中，this 的靜態型別是已知的，因此所選擇的（在編譯時）過載是 void IVisitor::visit(const Square&);。

• 對於 square.accept(visitor); 呼叫，通過 virtual 的動態排程用於知道要呼叫的 accept。

優點 ：

• 你可以通過新增新訪問者向 IShape 類新增新功能（SerializeAsXml，…）。

缺點 ：

• 新增新的具體形狀（Triangle，…）需要修改所有訪客。

將所有功能作為 virtual 方法放在 IShape 中的替代方案具有相反的優點和缺點：新增新功能需要修改所有現有形狀，但新增新形狀不會影響現有類。