子类和父类之间类型转换

1. 子类 -> 父类

父类 -> Base

#include<isotream>
using namespace std;

class Base{
public:
	Base() : pub_att(1){}
	int pub_att;
};

public继承的子类1 -> Derived1

class Derived1 : public Base{
public: 
	Derived1() : att(2){}
	int att;
};

private继承的子类2 -> Derived2

class Derived2 : private Base{
public: 
	Derived2() : att(3){}
	int att;
};

需要类型转换的函数 -> class_con

void class_con(Base & A_Base){
	cout << "class_con is called!" << endl;
	cout << "Base::pub_att = " << A_Base.pub_att << endl;
}

声明对象

int main(){
	Derived1 A_Derived1;
	Derived2 A_Derived2;

	Base A_Base1, A_Base2;

对象转型

public继承的子类对象自动向上转型：

	A_Base1 = A_Derived1;

private继承的子类对象不可转型：

	//A_Base2 = (Base)A_Derived2;

指针和引用转型

public继承的对象指针和引用自动向上转型：

	Base * base_ptr1 = &A_Derived1; //指针
	class_con(A_Derived1); //引用

private继承的对象指针和引用普通强制转型：

	Base * base_ptr2 = (Base *)&A_Derived2; //指针
	class_con((Base &)A_Derived2); //引用
}

输出：

class_con is called!
Base::pub_att = 1
class_con is called!
Base::pub_att = 1

1. 父类型指针和引用还是会访问父类型的成员！
2. private继承的对象指针和引用，父类的成员子类全部继承为private，普通强制转型后，将恢复为原来的public，所以使用class_on函数可以访问父类（见1）的pub_att。

2. 父类 -> 子类

#include<iostream>
using namespace std;

父类

class Base{
private:
	int pri_att;
public:
	Base() : pri_att(1){}
};

子类

class Derived : public Base{
public: 
	int Derived_att;
	Derived() : Derived_att(2) {}
};

传指针，传引用，传值

void class_cast_ptr(Derived * A_Derived){
	cout << "A_Derived->Derived_att = "
		 << A_Derived->Derived_att << endl;
}

void class_cast_ref(Derived & A_Derived){
	cout << "A_Derived.Derived_att = "
		<< A_Derived.Derived_att << endl;
}

void class_cast_val(Derived A_Derived){
	cout << "A_Derived.Derived_att = "
		<< A_Derived.Derived_att << endl;
	
};

int main(){

	Base A_Base;

对象转型

任何对象不可向下转型

	//class_cat_val(static_cast<Derived>(A_Base));

指针和引用转型

父类对象指针static_cast强制向下转型：

	Base * base_ptr = new Derived; //  public继承的子类对象指针自动向上转型
	class_cast_ptr(static_cast<Derived *>(base_ptr));

父类对象引用static_cast强制向下转型：

	class_cast_ref(static_cast<Derived &>(A_Base));
}

输出：

A_Derived->Derived_att = 2
A_Derived.Derived_att = 2096008816

1. 子类对象指针先向上转型，又向下转型，所以子类对象有值（子类指针访问子类成员），A_Derived->Derived_att = 2！！
2. 父类对象指针直接向下转型，所以子类对象无值，不可知，A_Derived.Derived_att = 2096008816