10.单继承和多继承的虚函数表结构-灵析社区

编译器将虚函数表的指针放在类的实例对象的内存空间中，该对象调用该类的虚函数时，通过指针找到虚函数表，根据虚函数表中存放的虚函数的地址找到对应的虚函数。
如果派生类没有重新定义基类的虚函数 A，则派生类的虚函数表中保存的是基类的虚函数 A 的地址，也就是说基类和派生类的虚函数 A 的地址是一样的。
如果派生类重写了基类的某个虚函数 B，则派生的虚函数表中保存的是重写后的虚函数 B 的地址，也就是说虚函数 B 有两个版本，分别存放在基类和派生类的虚函数表中。
如果派生类重新定义了新的虚函数 C，派生类的虚函数表保存新的虚函数 C 的地址。

1.单继承无虚函数覆盖的情况：

C++

#include <iostream>
#include <memory>
using namespace std;
 
 
typedef void (*func)(void);

#include <iostream>
using namespace std;

class Base
{
public:
    virtual void B_fun1() { cout << "Base::B_fun1()" << endl; }
    virtual void B_fun2() { cout << "Base::B_fun2()" << endl; }
    virtual void B_fun3() { cout << "Base::B_fun3()" << endl; }
};

class Derive : public Base
{
public:
    virtual void D_fun1() { cout << "Derive::D_fun1()" << endl; }
    virtual void D_fun2() { cout << "Derive::D_fun2()" << endl; }
    virtual void D_fun3() { cout << "Derive::D_fun3()" << endl; }
};

void printVtable(unsigned long *vptr, int offset) {
	func fn = (func)*((unsigned long*)(*vptr) + offset);
	fn();	
}

int main()
{
    Base *p = new Derive();
    p->B_fun1(); // Base::B_fun1()
	unsigned long* vPtr = (unsigned long*)(p);
	printVtable(vPtr, 0);
	printVtable(vPtr, 1);
	printVtable(vPtr, 2);
	printVtable(vPtr, 3);
	printVtable(vPtr, 3);
	printVtable(vPtr, 4);
    cout<<sizeof(Base)<<endl; // 8
    cout<<sizeof(Derive)<<endl; // 8
    return 0;
}
/*
Base::B_fun1()
Base::B_fun1()
Base::B_fun2()
Base::B_fun3()
Derive::D_fun1()
Derive::D_fun2()
Derive::D_fun3()
8
8
*/

基类和派生类的继承关系：

基类的虚函数表：

派生类的虚函数表：

虚函数按照声明顺序放在虚函数表里面。
父类的虚函数在子类的虚函数前面。

2.单继承有虚函数覆盖的情况：

C++

#include <iostream>
#include <memory>
using namespace std;
 
 
typedef void (*func)(void);

class Base
{
public:
    virtual void fun1() { cout << "Base::fun1()" << endl; }
    virtual void B_fun2() { cout << "Base::B_fun2()" << endl; }
    virtual void B_fun3() { cout << "Base::B_fun3()" << endl; }
};

class Derive : public Base
{
public:
    virtual void fun1() { cout << "Derive::fun1()" << endl; }
    virtual void D_fun2() { cout << "Derive::D_fun2()" << endl; }
    virtual void D_fun3() { cout << "Derive::D_fun3()" << endl; }
};

void printVtable(unsigned long *vptr, int offset) {
	func fn = (func)*((unsigned long*)(*vptr) + offset);
	fn();	
}

int main()
{
    Base *p = new Derive();
	unsigned long* vPtr = (unsigned long*)(p);
	printVtable(vPtr, 0);
	printVtable(vPtr, 1);
	printVtable(vPtr, 2);
	printVtable(vPtr, 3);
	printVtable(vPtr, 4);
    cout<<sizeof(Base)<<endl; // 8
    cout<<sizeof(Derive)<<endl; // 8
    return 0;
}

/*
Derive::fun1()
Base::B_fun2()
Base::B_fun3()
Derive::D_fun2()
Derive::D_fun3()
8
8
*/

派生类的虚函数表：

3.多继承无虚函数覆盖的情况：

C++

#include <iostream>
using namespace std;

class Base1
{
public:
    virtual void B1_fun1() { cout << "Base1::B1_fun1()" << endl; }
    virtual void B1_fun2() { cout << "Base1::B1_fun2()" << endl; }
    virtual void B1_fun3() { cout << "Base1::B1_fun3()" << endl; }
};
class Base2
{
public:
    virtual void B2_fun1() { cout << "Base2::B2_fun1()" << endl; }
    virtual void B2_fun2() { cout << "Base2::B2_fun2()" << endl; }
    virtual void B2_fun3() { cout << "Base2::B2_fun3()" << endl; }
};
class Base3
{
public:
    virtual void B3_fun1() { cout << "Base3::B3_fun1()" << endl; }
    virtual void B3_fun2() { cout << "Base3::B3_fun2()" << endl; }
    virtual void B3_fun3() { cout << "Base3::B3_fun3()" << endl; }
};

class Derive : public Base1, public Base2, public Base3
{
public:
    virtual void D_fun1() { cout << "Derive::D_fun1()" << endl; }
    virtual void D_fun2() { cout << "Derive::D_fun2()" << endl; }
    virtual void D_fun3() { cout << "Derive::D_fun3()" << endl; }
};

typedef void (*func)(void);

void printVtable(unsigned long *vptr, int offset) {
	func fn = (func)*((unsigned long*)(*vptr) + offset);
	fn();	
}

int main(){
    Base1 *p = new Derive();
    unsigned long* vPtr = (unsigned long*)(p);
	printVtable(vPtr, 0);
	printVtable(vPtr, 1);
	printVtable(vPtr, 2);
	printVtable(vPtr, 3);
	printVtable(vPtr, 4);
	printVtable(vPtr, 5);
	vPtr++;
	printVtable(vPtr, 0);
	printVtable(vPtr, 1);
	printVtable(vPtr, 2);
	vPtr++;
	printVtable(vPtr, 0);
	printVtable(vPtr, 1);
	printVtable(vPtr, 2);
    cout<<sizeof(Base1)<<endl; // 8
	cout<<sizeof(Base2)<<endl; // 8
	cout<<sizeof(Base3)<<endl; // 8
    cout<<sizeof(Derive)<<endl; // 8
    return 0;
}

/*
Base1::B1_fun1()
Base1::B1_fun2()
Base1::B1_fun3()
Derive::D_fun1()
Derive::D_fun2()
Derive::D_fun3()
Base2::B2_fun1()
Base2::B2_fun2()
Base2::B2_fun3()
Base3::B3_fun1()
Base3::B3_fun2()
Base3::B3_fun3()
8
8
8
24
*/

派生类的虚函数表：（基类的顺序和声明的顺序一致）

4.多继承有虚函数覆盖的情况：

C++

#include <iostream>
using namespace std;

class Base1
{
public:
    virtual void fun1() { cout << "Base1::fun1()" << endl; }
    virtual void B1_fun2() { cout << "Base1::B1_fun2()" << endl; }
    virtual void B1_fun3() { cout << "Base1::B1_fun3()" << endl; }
};
class Base2
{
public:
    virtual void fun1() { cout << "Base2::fun1()" << endl; }
    virtual void B2_fun2() { cout << "Base2::B2_fun2()" << endl; }
    virtual void B2_fun3() { cout << "Base2::B2_fun3()" << endl; }
};
class Base3
{
public:
    virtual void fun1() { cout << "Base3::fun1()" << endl; }
    virtual void B3_fun2() { cout << "Base3::B3_fun2()" << endl; }
    virtual void B3_fun3() { cout << "Base3::B3_fun3()" << endl; }
};

class Derive : public Base1, public Base2, public Base3
{
public:
    virtual void fun1() { cout << "Derive::fun1()" << endl; }
    virtual void D_fun2() { cout << "Derive::D_fun2()" << endl; }
    virtual void D_fun3() { cout << "Derive::D_fun3()" << endl; }
};

typedef void (*func)(void);

void printVtable(unsigned long *vptr, int offset) {
	func fn = (func)*((unsigned long*)(*vptr) + offset);
	fn();	
}

int main(){
    Base1 *p1 = new Derive();
    Base2 *p2 = new Derive();
    Base3 *p3 = new Derive();
    p1->fun1(); // Derive::fun1()
    p2->fun1(); // Derive::fun1()
    p3->fun1(); // Derive::fun1()
    unsigned long* vPtr = (unsigned long*)(p1);
	printVtable(vPtr, 0);
	printVtable(vPtr, 1);
	printVtable(vPtr, 2);
	printVtable(vPtr, 3);
	printVtable(vPtr, 4);
	vPtr++;
	printVtable(vPtr, 0);
	printVtable(vPtr, 1);
	printVtable(vPtr, 2);
	vPtr++;
	printVtable(vPtr, 0);
	printVtable(vPtr, 1);
	printVtable(vPtr, 2);
    cout<<sizeof(Base1)<<endl; // 8
	cout<<sizeof(Base2)<<endl; // 8
	cout<<sizeof(Base3)<<endl; // 8
    cout<<sizeof(Derive)<<endl; // 8
    return 0;
}

/*
Derive::fun1()
Derive::fun1()
Derive::fun1()
Derive::fun1()
Base1::B1_fun2()
Base1::B1_fun3()
Derive::D_fun2()
Derive::D_fun3()
Derive::fun1()
Base2::B2_fun2()
Base2::B2_fun3()
Derive::fun1()
Base3::B3_fun2()
Base3::B3_fun3()
8
8
8
24
*/

基类和派生类的关系：

派生类的虚函数表：