c++ - 派生类的成员函数行为能否由其实例化的类决定？

Question

Rectangle 和 Triangle 类都派生自 Shape。我想添加另一个类 ComplexShape，它可以是任何特定的形状并附加其他形状。

我知道这个问题有一些简单的解决方法，比如在基类中声明一个变量来存储形状属性，但我对解决标题问题的方法更感兴趣。如果可能，我该如何定义 ComplexShape 的构造函数，以便 ComplexShape 使用用于初始化它的类的方法？

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

class Shape {
    public: virtual unsigned int getPointCount() const = 0;
};

class Rectangle : public Shape {
    public: unsigned int getPointCount() const { return 4; }
};

class Triangle : public Shape {
    public: unsigned int getPointCount() const { return 3; }
};

class ComplexShape : public Shape {
public:
    std::vector<std::shared_ptr<Shape>> children;

    //What Constructor comes here?

    unsigned int getPointCount() const {
        unsigned int i{ 0u };
        for(auto shape : children) i += shape->getPointCount();
        return i;
    }
};

int main() {
    Triangle triangle();
    ComplexShape arrow; //How do I initialize this as a rectangle?
    arrow.children.push_back(std::shared_ptr<Shape>(new Triangle()));
    std::cout << arrow.getPointCount();
    return 0;
};

Answer 1

您可以尝试使用构造函数中使用的初始化列表:http://www.cplusplus.com/reference/initializer_list/initializer_list/

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

class Shape {
public: virtual unsigned int getPointCount() const = 0;
};

class Rectangle : public Shape {
public: unsigned int getPointCount() const { return 4; }
};

class Triangle : public Shape {
public: unsigned int getPointCount() const { return 3; }
};

class ComplexShape : public Shape {
public:
    std::vector<std::shared_ptr<Shape>> children;
    ComplexShape() {}

    ComplexShape(std::initializer_list<std::shared_ptr<Shape>> init) : children(init)
    {
        // do some dynamic_cast magic here
    }

    unsigned int getPointCount() const {
        unsigned int i{ 0u };
        for (auto shape : children) i += shape->getPointCount();
        return i;
    }
};

int main() {
    Triangle triangle();
    ComplexShape arrow; //How do I initialize this as a rectangle?
    arrow.children.push_back(std::shared_ptr<Shape>(new Triangle()));
    std::cout << arrow.getPointCount();

    // This could be simplified probably
    ComplexShape rect = { std::shared_ptr<Shape>(new Triangle()), std::shared_ptr<Shape>(new Triangle()) };
    return 0;
};