c++-多态和vptr指针
多态的原理
#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
using namespace std;
class Parent
{
public:
Parent(int a) {
this->a = a;
}
virtual void func(int a)
{
cout << "Parent::func(int)..." << endl;
}
virtual void func(int a, int b, int c)
{
cout << "Parent::func(int ,int ,int )...." << endl;
}
private:
int a;
};
class Child :public Parent
{
public:
Child(int a, int b) :Parent(a)
{
this->b = b;
}
virtual void func(int a)
{
cout << "Child: func(int)..." << endl;
}
void func(int a, int b) {
cout << "Child :func(int ,int )..." << endl;
}
virtual void func(int a, int b, int c)
{
cout << "Child ::func(int ,int ,int )..." << endl;
}
private:
int b;
};
void myFunc(Parent *pp)
{
pp->func(10);
}
int main(void)
{
//Parent *pp = new Parent(10);
//Parent *cp = new Child(100, 200);
Parent *pp = new Child(100, 200);
pp->func(10);//Parent ? Child
//如果调用一个普通函数,编译器根本就不会查找虚函数表。
//只有你调用的函数,是虚函数的时候,才会去查找虚函数表
// myFunc(pp);
pp->func(10, 20, 30);
return 0;
}
验证vptr指针的存在
#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
using namespace std;
class Parent
{
public:
virtual void func()
{
cout << "Parent::func().." << endl;
}
virtual void func(int a)
{
cout << "Parent::func().." << endl;
}
private:
int a;
};
class Parent2
{
public:
void func()
{
cout << "Parent2::func().." << endl;
}
private:
int a;
};
int main(void)
{
Parent p1;
Parent2 p2;
cout << "sizeof(p1) " << sizeof(p1) << endl;//多出来的4个字节就是vptr指针所占用的空间。
cout << "sizeof(p2) " << sizeof(p2) << endl;
return 0;
}
vptr指针分布初始化
#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
using namespace std;
class Parent
{
public:
Parent(int a)
{
cout << "Parent(int ..)" << endl;
this->a = a;
//print();//是调用父类的print() 还是 子类的print()?
//通过结果 此时调用的父类的print 并不是子类的print
}
virtual void print()
{
cout << "Parent::print()...a = "<<a << endl;
}
private:
int a;
};
class Child :public Parent
{
public:
Child(int a, int b) :Parent(a) //在调用父类的构造器的时候,会将vptr指针当做父类来处理。
//此时会临时指向父类的虚函数表
{
//将子类对象的空间有编程子类对象处理,vptr指针就从指向父类的表 变成 指向子类的表
cout << "Child (int ,int )" << endl;
this->b = b;
print();//此时vptr指针已经回到了 子类的表, 调用的是子类的print函数。
}
virtual void print() {
cout << "Child ::Print()..b = " << b << endl;
}
private:
int b;
};
int main(void)
{
Parent *pp = new Child(10, 20);
// pp->print();//发生多态
delete pp;
return 0;
}
父类指针和子类指针的步长
#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
using namespace std;
class Parent
{
public:
Parent(int a)
{
this->a = a;
}
virtual void print()
{
cout << "Parent::print a= " << a << endl;
}
int a;
};
class Child :public Parent
{
public:
Child(int a) :Parent(a)
{
}
virtual void print()
{
cout << "Child::print a= " << a << endl;
}
int b;
};
int main(void)
{
Child array[] = { Child(0), Child(1), Child(2) };
Parent *pp = &array[0];
Child *cp = &array[0];
pp++;
pp->print();
cp->print();
#if 0
pp++;//pp +sizeof(Parent)
cp++;//cp +sizeof(Child)
pp->print();
cp->print();
#endif
cout << "-----" << endl;
int i = 0;
for (cp= &array[0], i = 0; i < 3; i++, cp++) {
cp->print();
}
return 0;
}
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