---- 整理自狄泰软件唐佐林老师课程
#include
#include
using namespace std;
class A
{
int i;
int j;
char c;
double d;
public:
void print()
{
cout << "i = " << i << ", "
<< "j = " << j << ", "
<< "c = " << c << ", "
<< "d = " << d << endl;
}
};
struct B
{
int i;
int j;
char c;
double d;
};
int main()
{
A a;
cout << "sizeof(A) = " << sizeof(A) << endl; // 20 bytes
cout << "sizeof(a) = " << sizeof(a) << endl;
cout << "sizeof(B) = " << sizeof(B) << endl; // 20 bytes
a.print();
B* p = reinterpret_cast<B*>(&a);
p->i = 1;
p->j = 2;
p->c = 'c';
p->d = 3;
a.print();
p->i = 100;
p->j = 200;
p->c = 'C';
p->d = 3.14;
a.print();
return 0;
}
运行时的对象退化为 结构体 的形式
所有成员变量在内存中 依次排布
成员变量间 可能存在内存空隙
可以通过对象的内存地址直接访问成员变量
访问权限 在运行时失效
访问权限在编译时有效,一旦编译通过,运行时就可以通过指针修改成员变量的值
类中的成员函数位于代码段中
调用成员函数时 对象地址 作为参数 隐式传递(this指针)
成员函数通过 对象地址 访问成员变量
C++语法规则 隐藏了 对象地址的传递过程
#include
#include
using namespace std;
class Demo
{
int mi;
int mj;
public:
Demo(int i, int j)
{
mi = i;
mj = j;
}
int getI()
{
return mi;
}
int getJ()
{
return mj;
}
int add(int value)
{
return mi + mj + value;
}
};
int main()
{
Demo d(1, 2);
cout << "sizeof(d) = " << sizeof(d) << endl;
cout << "d.getI() = " << d.getI() << endl;
cout << "d.getJ() = " << d.getJ() << endl;
cout << "d.add(3) = " << d.add(3) << endl;
return 0;
}
#include "50-2.h"
#include "malloc.h"
struct ClassDemo
{
int mi;
int mj;
};
Demo* Demo_Create(int i, int j)
{
struct ClassDemo* ret = (struct ClassDemo*)malloc(sizeof(struct ClassDemo));
if( ret != NULL )
{
ret->mi = i;
ret->mj = j;
}
return ret;
}
int Demo_GetI(Demo* pThis)
{
struct ClassDemo* obj = (struct ClassDemo*)pThis;
return obj->mi;
}
int Demo_GetJ(Demo* pThis)
{
struct ClassDemo* obj = (struct ClassDemo*)pThis;
return obj->mj;
}
int Demo_Add(Demo* pThis, int value)
{
struct ClassDemo* obj = (struct ClassDemo*)pThis;
return obj->mi + obj->mj + value;
}
void Demo_Free(Demo* pThis)
{
free(pThis);
}
#include
#include
using namespace std;
class Demo
{
protected:
int mi;
int mj;
public:
virtual void print()
{
cout << "mi = " << mi << ", "
<< "mj = " << mj << endl;
}
};
class Derived : public Demo
{
int mk;
public:
Derived(int i, int j, int k)
{
mi = i;
mj = j;
mk = k;
}
void print()
{
cout << "mi = " << mi << ", "
<< "mj = " << mj << ", "
<< "mk = " << mk << endl;
}
};
struct Test
{
void* p;
int mi;
int mj;
int mk;
};
int main()
{
cout << "sizeof(Demo) = " << sizeof(Demo) << endl;
cout << "sizeof(Derived) = " << sizeof(Derived) << endl;
Derived d(1, 2, 3);
Test* p = reinterpret_cast<Test*>(&d);
cout << "Before changing ..." << endl;
d.print();
p->mi = 10;
p->mj = 20;
p->mk = 30;
cout << "After changing ..." << endl;
d.print();
return 0;
}
C++多态的实现原理
当类中声明虚函数时,编译器会在类中生成一个 虚函数表
虚函数表是一个 存储 成员函数地址 的数据结构
虚函数表是由编译器自动生成与维护的
virtual成员函数 会被编译器放入虚函数表中
存在虚函数时,每个对象 中都有一个 指向虚函数表的指针
/* demo.c */
#include "demo.h"
#include "malloc.h"
static int Demo_Virtual_add(Demo* pThis, int value);
static int Derived_Virtual_add(Demo* pThis, int value);
struct VTable // 2、定义虚函数表数据结构
{
int (*pAdd)(void*, int); // 3、虚函数表里面存储什么??
};
struct ClassDemo
{
struct VTable* vptr; // 1、定义虚函数表的指针==>虚函数表指针类型??
int mi;
int mj;
};
struct ClassDerived {
struct ClassDemo d;
int mk;
};
static struct VTable g_Demo_vtbl = {
Demo_Virtual_add
};
static struct VTable g_Derived_vtbl = {
Derived_Virtual_add
};
Demo* Demo_Create(int i, int j) {
struct ClassDemo* ret = \
(struct ClassDemo*)malloc(sizeof(struct ClassDemo));
if (ret != NULL) {
ret->vptr = &g_Demo_vtbl; // 4、关联对象和指向的具体的虚函数表
ret->mi = i;
ret->mj = j;
}
return ret;
}
int Demo_GetI(Demo* pThis) {
struct ClassDemo* obj = (struct ClassDemo*)pThis;
return obj->mi;
}
int Demo_GetJ(Demo* pThis) {
struct ClassDemo* obj = (struct ClassDemo*)pThis;
return obj->mj;
}
// 6、定义虚函数表中指针所指向的具体函数
static int Demo_Virtual_add(Demo* pThis, int value) {
struct ClassDemo* obj = (struct ClassDemo*)pThis;
return obj->mi + obj->mj + value;
}
// 5、分析具体的虚函数!!!
int Demo_Add(Demo* pThis, int value) {
struct ClassDemo* obj = (struct ClassDemo*)pThis;
return obj->vptr->pAdd(pThis, value);
// 通过对象,找到指向虚函数表的指针,然后在虚函数表中找到具体要调用的函数地址
}
void Demo_Free(Demo* pThis) {
free(pThis);
}
Derived* Derived_Create(int i, int j, int k) {
struct ClassDerived* ret = \
(struct ClassDerived*)malloc(sizeof(struct ClassDerived));
if( ret != NULL ) {
ret->d.vptr = &g_Derived_vtbl;
ret->d.mi = i;
ret->d.mj = j;
ret->mk = k;
}
return ret;
}
int Derived_GetK(Derived* pThis) {
struct ClassDerived* obj = (struct ClassDerived*)pThis;
return obj->mk;
}
static int Derived_Virtual_add(Demo* pThis, int value) {
struct ClassDerived* obj = (struct ClassDerived*)pThis;
return obj->mk + value;
}
int Derived_Add(Derived* pThis, int value) {
struct ClassDerived* obj = (struct ClassDerived*)pThis;
return obj->d.vptr->pAdd(pThis, value);
}
/* demo.h */
#ifndef _DEMO_H_
#define _DEMO_H_
typedef void Demo;
typedef void Derived;
Demo* Demo_Create(int i, int j);
int Demo_GetI(Demo* pThis);
int Demo_GetJ(Demo* pThis);
int Demo_Add(Demo* pThis, int value);
void Demo_Free(Demo* pThis);
Derived* Derived_Create(int i, int j, int k);
int Derived_GetK(Derived* pThis);
int Derived_Add(Derived* pThis, int value);
#endif
#include
#include "demo.h"
void run(Demo* p, int v)
{
int r = Demo_Add(p, v);
printf("r = %d\n", r);
}
int main()
{
Demo* pb = Demo_Create(1, 2);
Derived* pd = Derived_Create(1, 22, 333);
printf("pb->add(3) = %d\n", Demo_Add(pb, 3));
printf("pd->add(3) = %d\n", Derived_Add(pd, 3));
run(pb, 3);
run(pd, 3);
Demo_Free(pb);
Demo_Free(pd);
return 0;
}