多态二元函数

Question

我有三个形状类 Circle, Square, ConvexPolygon, 我有函数

bool ShapesIntersect(const Circle& s1, const Circle& s2);
bool ShapesIntersect(const Circle& s1, const Square& s2);
// ... and the other 7 combinations

我想要一个类似于

的多态函数

Matrix<bool> ComputeShapeIntersectionMatrix(
    const vector<Shape>& shapes1,
    const vector<Shape>& shapes2);

调用上面特定于形状的方法来填充交点矩阵。

我认为在 C++ 中不可能完全做到这一点，但我可以接受任何其他解决方案，只要我可以添加新的形状类型而不更改计算交集矩阵的代码即可。

Answer 1

我会这样解决：

struct Circle;
struct Polygon;
struct Square;

struct Intersector {
    virtual bool intersects(const Circle& ) const = 0;
    virtual bool intersects(const Polygon& ) const = 0;
    virtual bool intersects(const Square& ) const = 0;
};

template<class SHAPE>
struct ShapeIntersetor;

template<>
struct ShapeIntersetor<Square> : Intersector {
    const Square& square;
    ShapeIntersetor(const Square& square) : square(square) { }
    bool intersects(const Circle& ) const override { /* Intersection code */ }
    bool intersects(const Polygon& ) const override { /* Intersection code */ }
    bool intersects(const Square& ) const override { /* Intersection code */ }
};

// Repeat above for each shape type

struct Shape {
    virtual ~Shape() = default;
    virtual std::unique_ptr<Intersector> get_intersector() const = 0;
    virtual bool intersects(const Intersector& isector) const = 0;
};

struct Polygon : Shape {
    std::unique_ptr<Intersector> get_intersector() const override {
        return make_unique<ShapeIntersector<Polygon>>(*this);
    }
    bool intersects(const Intersector& isector) const override {
        return isector.intersects(*this);
    }
}; 

// Repeat above for each shape type

Answer 2

我做了一个。

如果使用std::variant，就变得简单了：

using Shape = std::variant<Circle, Square, ConvexPolygon>;

struct intersect
{
    template <typename Shape1, typename Shape2>
    bool operator()(const Shape1& lhs, const Shape2& rhs) const {
        return ShapesIntersect(lhs, rhs);
    }
};



Matrix<bool> ComputeShapeIntersectionMatrix(
    const vector<Shape>& shapes1,
    const vector<Shape>& shapes2)
{
    Matrix<bool> res(shapes1.size(), shapes2.size());

    for (std::size_t i = 0; i != shapes1.size(); ++i) {
        for (std::size_t j = 0; j != shapes2.size(); ++j) {
            res[i][j] = std::visit(intersect{}, shapes1[i], shapes2[j]);
        }
    }
    return res;
}

多态二元函数

Polymorphic binary function

c++

polymorphism

multiple-dispatch

double-dispatch