正在从iostream中读取超类的子类实例。>>运算符如何知道是哪个子类？

#include <iostream>

class SuperClass {
public:
    virtual ~SuperClass() {}
    virtual void print() {
        std::cout << "This is a SuperClass." << std::endl;
    }
};

class SubClassA : public SuperClass {
public:
    void print() override {
        std::cout << "This is a SubClassA." << std::endl;
    }
};

class SubClassB : public SuperClass {
public:
    void print() override {
        std::cout << "This is a SubClassB." << std::endl;
    }
};

int main() {
    SuperClass* obj;

    // 从iostream中读取SubClassA的实例
    obj = new SubClassA();
    std::cin >> *obj;
    obj->print();

    // 从iostream中读取SubClassB的实例
    obj = new SubClassB();
    std::cin >> *obj;
    obj->print();

    delete obj;
    return 0;
}