c++——dynamic_cast的使用

1）dynamic_cast要求操作数必须是多态类型（即要求父类要存在virtual虚函数）；

class Base
{
public:Base() {}~Base() {}void test(){cout << "Base" << endl;}
};class Derived : public Base
{
public:Derived(){}~Derived(){}void test(){cout << "Derived" << endl;}
};int main()
{Base* pB = new Derived();Derived* pD = dynamic_cast<Derived*>(pB);  // error: dynamic_cast要求操作数必须是多态类型system("pause");return 0;
}

2）单继承情况，可以判断dynamic_cast强转后的指针是否为空判断是否转换成功；

class Base
{
public:Base() {}~Base() {}virtual void test(){cout << "Base" << endl;}
};class Derived : public Base
{
public:Derived(){}~Derived(){}virtual void test(){cout << "Derived" << endl;}
};int main()
{// 1. 单继承  pD指向的是DerivedBase* pB = new Derived();Derived* pD = dynamic_cast<Derived*>(pB);if (nullptr == pD)cout << "pD is nullptr!" << endl;// 2. 单继承 pD1不是指向Derived 此时dynamic_cast强转后返回nullptrBase* pB1 = new Base();Derived* pD1 = dynamic_cast<Derived*>(pB1);  if (nullptr == pD1)cout << "pD1 is nullptr!" << endl;system("pause");return 0;
}

3）多继承；

class Base
{
public:Base() {}~Base() {}virtual void test(){cout << "Base" << endl;}
};class Base1
{
public:Base1() {}~Base1() {}virtual void test1(){cout << "Base1" << endl;}
};class Derived : public Base, public Base1
{
public:Derived(){}~Derived(){}virtual void test(){cout << "Derived" << endl;}virtual void test1(){cout << "Derived-1" << endl;}
};int main()
{// 1. 多继承  pD指向的是DerivedBase* pB = new Derived();Derived* pD = dynamic_cast<Derived*>(pB);if (nullptr == pD)cout << "pD is nullptr!" << endl;// 2. 多继承 pD1指向的是DerivedBase1* pD1 = dynamic_cast<Base1*>(pB);if (nullptr == pD1)cout << "pD1 is nullptr!" << endl;pD1->test1();  // 打印 Derived-1system("pause");return 0;
}

4）基类强转为派生类，此种方式无效，dynamic_cast返回的指针为nullptr；

class Base
{
public:Base() {}~Base() {}virtual void test(){cout << "Base" << endl;}
};class Base1
{
public:Base1() {}~Base1() {}virtual void test1(){cout << "Base1" << endl;}
};class Derived : public Base, public Base1
{
public:Derived(){}~Derived(){}virtual void test(){cout << "Derived" << endl;}virtual void test1(){cout << "Derived-1" << endl;}
};int main()
{Base* pB1 = new Base();Derived* pD2 = dynamic_cast<Derived*>(pB1);  // 转换无效 pD2为nullptrif (nullptr == pD2)cout << "pD2 is nullptr!" << endl;system("pause");return 0;
}

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