106 lines
2.4 KiB
C++
106 lines
2.4 KiB
C++
/**
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* @file vptr1.cpp
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* @brief C++虚函数vptr和vtable
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* 编译：g++ -g -o vptr vptr1.cpp -std=c++11
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* @author 光城
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* @version v1
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* @date 2019-07-20
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*/
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#include <iostream>
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#include <stdio.h>
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using namespace std;
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/**
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* @brief 函数指针
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*/
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typedef void (*Fun)();
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/**
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* @brief 基类
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*/
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class Base
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{
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public:
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Base(){};
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virtual void fun1()
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{
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cout << "Base::fun1()" << endl;
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}
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virtual void fun2()
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{
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cout << "Base::fun2()" << endl;
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}
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virtual void fun3(){}
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~Base(){};
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};
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/**
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* @brief 派生类
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*/
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class Derived: public Base
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{
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public:
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Derived(){};
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void fun1()
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{
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cout << "Derived::fun1()" << endl;
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}
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void fun2()
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{
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cout << "DerivedClass::fun2()" << endl;
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}
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~Derived(){};
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};
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/**
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* @brief 获取vptr地址与func地址,vptr指向的是一块内存，这块内存存放的是虚函数地址，这块内存就是我们所说的虚表
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*
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* @param obj
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* @param offset
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*
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* @return
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*/
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Fun getAddr(void* obj,unsigned int offset)
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{
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cout<<"======================="<<endl;
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void* vptr_addr = (void *)*(unsigned long *)obj; //64位操作系统，占8字节，通过*(unsigned long *)obj取出前8字节，即vptr指针
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printf("vptr_addr:%p\n",vptr_addr);
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/**
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* @brief 通过vptr指针访问virtual table，因为虚表中每个元素(虚函数指针)在64位编译器下是8个字节，因此通过*(unsigned long *)vptr_addr取出前8字节，
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* 后面加上偏移量就是每个函数的地址！
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*/
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void* func_addr = (void *)*((unsigned long *)vptr_addr+offset);
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printf("func_addr:%p\n",func_addr);
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return (Fun)func_addr;
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}
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int main(void)
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{
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Base ptr;
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Derived d;
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Base *pt = new Derived(); // 基类指针指向派生类实例
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Base &pp = ptr; // 基类引用指向基类实例
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Base &p = d; // 基类引用指向派生类实例
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cout<<"基类对象直接调用"<<endl;
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ptr.fun1();
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cout<<"基类引用指向基类实例"<<endl;
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pp.fun1();
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cout<<"基类指针指向派生类实例并调用虚函数"<<endl;
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pt->fun1();
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cout<<"基类引用指向派生类实例并调用虚函数"<<endl;
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p.fun1();
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// 手动查找vptr 和 vtable
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Fun f1 = getAddr(pt, 0);
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(*f1)();
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Fun f2 = getAddr(pt, 1);
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(*f2)();
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delete pt;
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return 0;
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}
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