dynamic_cast is typically used for down-cast check. e.g.

class Base{
// .... 
// Base must be polymorphic!!! (has virtual members)
virtual void foo();

};

class Derive: public Base{
virtual void foo() override {
cout<<"Derive::foo()"<<endl;
}

void derive_only() {
cout<<"this method only exists for derive type\n";
}

};
Base* pDerive = new Derive();
Derive* pd;
if((pd = dynamic_cast<Derive*>(pDerive)){
//downcast successful 
pd->derive_only(); // prints "derive only method"
}else{
//the pDerive is not Derive type
}

dynamic_cast具有类型检查的功能，失败会抛出bad_cast异常。比static_cast更安全，static_cast是不作任何检查直接强转的

经过我反复试验, 发现只要在父类中加入一个虚函数, 或者把析构函数设为虚函数就可以了, 但是并不知道为什么会这样...

最简单的例子:

class Father
{
public:
    virtual void func1() { std::cout << "Father" << std::endl; }
    virtual ~Father(){}
};
class Son :public Father
{
public:
    virtual void func1(){ std::cout << "Son1" << std::endl; }
    void func2(){ std::cout << "Son2" << std::endl; }
    virtual ~Son(){}
};
class Factory
{
private:
    Father* myFather;
public:
    void setFun(Father* m_Father) { myFather = m_Father; }
    Father* getFun() { return myFather;};
};

int main()
{
    Father *m_Father = new Father();
    Factory* m_Factory = new Factory();
    m_Factory->setFun(m_Father );
    m_Factory->getFun()->func1();
    dynamic_cast<Son*>(m_Factory->getFun())->func2();
    ///
    return 0;
}

dynamic_cast 转换的依据是虚表前面的 type_info 进行转换，只有包含虚函数的类才可以使用 dynamic_cast 。

而且上述的 type_info 是在编译期由编译器生成，所以在编译的时候，如果没有找到 type_info，就会报错。