c/c++ 多态的实现原理分析

多态的实现原理分析

当类里有一个函数被声明成虚函数后，创建这个类的对象的时候，就会自动加入一个__vfptr的指针，

__vfptr维护虚函数列表。如果有三个虚函数，则__vfptr指向的是第一个虚函数，

__vfptr+1指向的是第二个虚函数，__vfptr+2指向的是第三个虚函数。

当子类覆盖了父类的虚函数后，__vfptr+n就不是指向父类的虚函数了，而是指向的子类的函数。

所以当我们用父类的指针或者引用去调用被覆盖的虚函数时，才能够调用到子类的函数。

1.成员函数加上virtual修饰后，sizeof（类）的数字变大，即使有多个成员函数声明成虚函数，sizeof（类）的数字不会再变大。

加了virtual后，用GDB调试的时候，发现对象b下多了个_vptr

#include <iostream>
using namespace std;

class Base{
public:
  Base() : x(0){

  }
  ~Base(){

  }
  virtual void show(){
    cout << "Base show" << endl;
  }
private:
  int x;
};

int main(){
  cout << sizeof(Base) << endl;//16
  Base b;
}

没加virtual的效果：

#include <iostream>
using namespace std;

class Base{
public:
  Base() : x(0){

  }
  ~Base(){

  }
  void show(){
    cout << "Base show" << endl;
  }
private:
  int x;
};

int main(){
  cout << sizeof(Base) << endl;//4
  Base b;
}

2.声明了虚拟函数后，系统会自动维护一个指针_vptr，指向虚函数列表。

多态是通过虚函数列表实现的。

#include <iostream>
using namespace std;

class Base{
public:
  Base() : x(0){

  }
  ~Base(){

  }
  virtual void show(){
    cout << "Base show" << endl;
  }
  virtual void print(){
    cout << "Base print" << endl;
  }
  void fun(){
    cout << "Base fun" << endl;
  }
  
private:
  int x;
};

class D : public Base{
public:
  D() : y(0){

  }
  ~D(){

  }
  void show(){
    cout << "D show" << endl;
  }
  void fun(){
    cout << "D fun" << endl;
  }
  virtual void list(){
    cout << "D list" << endl;
  }
private:
  int y;
};
int main(){
  D d;
  d.fun();//调用子类的fun方法，父类的fun方法被隐藏了

  Base *pb = &d;
  pb->show();//调用子类的show方法，多态（覆盖）
  pb->fun();//调用父类的fun方法

  Base &fb = d;
  fb.show();//用子类的show方法，多态（覆盖）
  fb.fun();//调用父类的fun方法
}

3.虚函数表

3.1 单继承，无覆盖

#include <iostream>
using namespace std;

class Base{
public:
  virtual void f(){}
  virtual void g(){}
  virtual void h(){}  
};

class D : public Base{
  virtual void f1(){}
  virtual void g1(){}
  virtual void h1(){}  
};

int main(){
  D d;
}

用GDB查看函数指针的方法：x/a 内存地址。查看下一个函数指针：x/a 内存地址+8

gdb的分析结果：

18	  D d;
(gdb) n
(gdb) p d
$2 = {<Base> = {_vptr.Base = 0x555555755d48 <vtable for D+16>}, <No data fields>}
(gdb) x/a 0x555555755d48
$3 = (long *) 0x555555554a06 <Base::f()>//虚函数表第1个地址
(gdb) x/a 0x555555755d48+8
$4 = (long *) 0x555555554a12 <Base::g()>//虚函数表第2个地址
(gdb) x/a 0x555555755d48+16
$5 = (long *) 0x555555554a1e <Base::h()>//虚函数表第3个地址
(gdb) x/a 0x555555755d48+24
$6 = (long *) 0x555555554a2a <D::f1()>//虚函数表第4个地址
(gdb) x/a 0x555555755d48+32
$7 = (long *) 0x555555554a36 <D::g1()>//虚函数表第5个地址
(gdb) x/a 0x555555755d48+40
$8 = (long *) 0x555555554a42 <D::h1()>//虚函数表第6个地址
(gdb) x/a 0x555555755d48+48
$9 = (long *) 0x7ffff7dc7438 <vtable for __cxxabiv1::__si_class_type_info+16>

3.2 单继承，覆盖

#include <iostream>
using namespace std;

class Base{
public:
  virtual void f(){}
  virtual void g(){}
  virtual void h(){}  
};

class D : public Base{
  virtual void f(){}
  virtual void g1(){}
  virtual void h1(){}  
};

int main(){
  D d;
}

gdb的分析结果：

18	  D d;
(gdb) n
(gdb) p d
$1 = {<Base> = {_vptr.Base = 0x555555755d50 <vtable for D+16>}, <No data fields>}
(gdb) p (long*)*((long*)0x555555755d50)
$2 = (long *) 0x555555554a0e <D::f()>//虚函数表第1个地址(变成了子类的f())
(gdb) p (long*)*((long*)0x555555755d50+1)
$3 = (long *) 0x5555555549f6 <Base::g()>//虚函数表第2个地址
(gdb) p (long*)*((long*)0x555555755d50+2)
$4 = (long *) 0x555555554a02 <Base::h()>//虚函数表第3个地址
(gdb) p (long*)*((long*)0x555555755d50+3)
$5 = (long *) 0x555555554a1a <D::g1()>//虚函数表第4个地址
(gdb) p (long*)*((long*)0x555555755d50+4)
$6 = (long *) 0x555555554a26 <D::h1()>//虚函数表第5个地址
(gdb) p (long*)*((long*)0x555555755d50+5)
$7 = (long *) 0x7ffff7dc7438 <vtable for __cxxabiv1::__si_class_type_info+16>

3.3 多继承，不覆盖

#include <iostream>
using namespace std;

class Base{
public:
  virtual void f(){}
  virtual void g(){}
  virtual void h(){}  
};
class Base1{
public:
  virtual void f(){}
  virtual void g(){}
  virtual void h(){}  
};
class Base2{
public:
  virtual void f(){}
  virtual void g(){}
  virtual void h(){}  
};
class D : public Base,public Base1,public Base2{
  virtual void f1(){}
  virtual void g1(){}
  virtual void h1(){}  
};
int main(){
  D d;
}

gdb的分析结果：

29	  D d;
(gdb) n
(gdb) p d
$1 = {<Base> = {_vptr.Base = 0x555555755ca8 <vtable for D+16>}, <Base1> = {
    _vptr.Base1 = 0x555555755ce8 <vtable for D+80>}, <Base2> = {
    _vptr.Base2 = 0x555555755d10 <vtable for D+120>}, <No data fields>}
(gdb) p (long*)*((long*)0x555555755ca8)
$3 = (long *) 0x555555554b6c <Base::f()>//虚函数表(Base)第1个地址
(gdb) p (long*)*((long*)0x555555755ca8+1)
$4 = (long *) 0x555555554b78 <Base::g()>//虚函数表(Base)第2个地址
(gdb) p (long*)*((long*)0x555555755ca8+2)
$5 = (long *) 0x555555554b84 <Base::h()>//虚函数表(Base)第3个地址
(gdb) p (long*)*((long*)0x555555755ca8+3)
$6 = (long *) 0x555555554bd8 <D::f1()>//虚函数表(Base)第4个地址
(gdb) p (long*)*((long*)0x555555755ca8+4)
$7 = (long *) 0x555555554be4 <D::g1()>//虚函数表(Base)第5个地址
(gdb) p (long*)*((long*)0x555555755ca8+5)
$8 = (long *) 0x555555554bf0 <D::h1()>//虚函数表(Base)第6个地址
(gdb) p (long*)*((long*)0x555555755ca8+6)
$9 = (long *) 0xfffffffffffffff8
(gdb) p (long*)*((long*)0x555555755ce8)
$10 = (long *) 0x555555554b90 <Base1::f()>//虚函数表(Base1)第1个地址
(gdb) p (long*)*((long*)0x555555755ce8+1)
$11 = (long *) 0x555555554b9c <Base1::g()>//虚函数表(Base1)第2个地址
(gdb) p (long*)*((long*)0x555555755ce8+2)
$12 = (long *) 0x555555554ba8 <Base1::h()>//虚函数表(Base1)第3个地址
(gdb) p (long*)*((long*)0x555555755ce8+3)
$13 = (long *) 0xfffffffffffffff0
(gdb) p (long*)*((long*)0x555555755d10)
$14 = (long *) 0x555555554bb4 <Base2::f()>//虚函数表(Base2)第1个地址
(gdb) p (long*)*((long*)0x555555755d10+1)
$15 = (long *) 0x555555554bc0 <Base2::g()>//虚函数表(Base2)第2个地址
(gdb) p (long*)*((long*)0x555555755d10+2)
$16 = (long *) 0x555555554bcc <Base2::h()>//虚函数表(Base2)第3个地址
(gdb) p (long*)*((long*)0x555555755d10+3)
$17 = (long *) 0x7ffff7dc74f8 <vtable for __cxxabiv1::__vmi_class_type_info+16>

3.4 多继承，覆盖

#include <iostream>
using namespace std;

class Base{
public:
  virtual void f(){}
  virtual void g(){}
  virtual void h(){}  
};
class Base1{
public:
  virtual void f(){}
  virtual void g(){}
  virtual void h(){}  
};
class Base2{
public:
  virtual void f(){}
  virtual void g(){}
  virtual void h(){}  
};
class D : public Base,public Base1,public Base2{
  virtual void f(){}
  virtual void g1(){}
  virtual void h1(){}  
};
int main(){
  D d;
}

gdb的分析结果：

29	  D d;
(gdb) n
(gdb) p d
$1 = {<Base> = {_vptr.Base = 0x555555755cb0 <vtable for D+16>}, <Base1> = {
    _vptr.Base1 = 0x555555755ce8 <vtable for D+72>}, <Base2> = {
    _vptr.Base2 = 0x555555755d10 <vtable for D+112>}, <No data fields>}
(gdb) p (long*)*((long*)0x555555755cb0)
$2 = (long *) 0x555555554ba4 <D::f()>//虚函数表(Base)第1个地址(变成了子类的f())
(gdb) p (long*)*((long*)0x555555755cb0+1)
$3 = (long *) 0x555555554b5c <Base::g()>//虚函数表(Base)第2个地址
(gdb) p (long*)*((long*)0x555555755cb0+2)
$4 = (long *) 0x555555554b68 <Base::h()>//虚函数表(Base)第3个地址
(gdb) p (long*)*((long*)0x555555755cb0+3)
$5 = (long *) 0x555555554bbc <D::g1()>//虚函数表(Base)第4个地址
(gdb) p (long*)*((long*)0x555555755cb0+4)
$6 = (long *) 0x555555554bc8 <D::h1()>//虚函数表(Base)第5个地址
(gdb) p (long*)*((long*)0x555555755cb0+5)
$7 = (long *) 0xfffffffffffffff8
(gdb) p (long*)*((long*)0x555555755ce8)
$8 = (long *) 0x555555554bb5 <non-virtual thunk to D::f()>//虚函数表(Base1)第1个地址(变成了子类的f())
(gdb) p (long*)*((long*)0x555555755ce8+1)
$9 = (long *) 0x555555554b74 <Base1::g()>//虚函数表(Base1)第2个地址
(gdb) p (long*)*((long*)0x555555755ce8+2)
$10 = (long *) 0x555555554b80 <Base1::h()>//虚函数表(Base1)第3个地址
(gdb) p (long*)*((long*)0x555555755ce8+3)
$11 = (long *) 0xfffffffffffffff0
(gdb) p (long*)*((long*)0x555555755d10)
$12 = (long *) 0x555555554baf <non-virtual thunk to D::f()>//虚函数表(Base2)第1个地址(变成了子类的f())
(gdb) p (long*)*((long*)0x555555755d10+1)
$13 = (long *) 0x555555554b8c <Base2::g()>//虚函数表(Base2)第2个地址
(gdb) p (long*)*((long*)0x555555755d10+2)
$14 = (long *) 0x555555554b98 <Base2::h()>//虚函数表(Base2)第3个地址
(gdb) p (long*)*((long*)0x555555755d10+3)
$15 = (long *) 0x7ffff7dc74f8 <vtable for __cxxabiv1::__vmi_class_type_info+16>