虛函數表指針：vfptr -> 指向一個虛函數表，為了解決多态問題。
虛基類指針：vbptr -> 指向一個虛基類表，為了解決類似棱形繼承的二義性問題。

虛繼承

#include <iostream>

using namespace std;

class Animal
{
public:
    virtual ~Animal() = default;
    int animal;
    virtual void f() { cout << "Animal::f()" << endl; }
    virtual void g() { cout << "Animal::g()" << endl; }

};
//這裡添加的virtual對于Sheep和Tuo本身沒影響，隻對他們的子類有影響。
class Sheep : virtual public Animal
{
public:
    int sheep;
    virtual void f() { cout << "sheep::f()" << endl; }
    virtual void g() { cout << "sheep::g()" << endl; }
    virtual void h() { cout << "sheep::h()" << endl; }
};

class Tuo : virtual public Animal
{
public:
    int tuo;
    virtual void f() { cout << "Tuo::f()" << endl; }
    virtual void g() { cout << "Tuo::g()" << endl; }
    virtual void h() { cout << "Tuo::h()" << endl; }
};

class SheepTuo :public Sheep, public Tuo
{
public:
    int sheep_tuo;
    virtual void f() { cout << "SheepTuo::f()" << endl; }
    virtual void g() { cout << "SheepTuo::g()" << endl; }
    virtual void h() { cout << "SheepTuo::h()" << endl; }
};

int main()
{
    SheepTuo st;
    cout << "sizeof(SheepTuo) is " << sizeof(SheepTuo) << endl;
    return 0;
}

繼承關系圖

虛函數和虛繼承虛繼承帶虛函數的多重繼承的記憶體布局參考

打開工具->Visual studio指令提示工具，cd 到源碼所在的路徑下，輸入cl /d1reportSingleClassLayout"要檢視的類名" “檔案名”，在這裡就是cl /d1reportSingleClassLayoutSheepTuo main.cpp。可以看到目前類記憶體的結構。（編譯後才能檢視到記憶體分布）。

Animal的記憶體布局：

class Animal    size(8):
        +---
 0      | {vfptr}
 4      | animal
        +---

Animal::[email protected]:
        | &Animal_meta
        |  0
 0      | &Animal::{dtor}
 1      | &Animal::f
 2      | &Animal::g

Animal::{dtor} this adjustor: 0
Animal::f this adjustor: 0
Animal::g this adjustor: 0
Animal::__delDtor this adjustor: 0
Animal::__vecDelDtor this adjustor: 0

Sheep的記憶體布局（虛繼承會多出一個vbptr，并且自身的類布局會放到最前面,進而會有自身的vfptr）

class Sheep     size(20):
        +---
 0      | {vfptr}
 4      | {vbptr}
 8      | sheep
        +---
        +--- (virtual base Animal)
12      | {vfptr}
16      | animal
        +---

Sheep::[email protected]@:
        | &Sheep_meta
        |  0
 0      | &Sheep::h

Sheep::[email protected]:
 0      | -4
 1      | 8 (Sheepd(Sheep+4)Animal)

Sheep::[email protected]@:
        | -12
 0      | &Sheep::{dtor}
 1      | &Sheep::f
 2      | &Sheep::g

Sheep::{dtor} this adjustor: 12
Sheep::f this adjustor: 12
Sheep::g this adjustor: 12
Sheep::h this adjustor: 0
Sheep::__delDtor this adjustor: 12
Sheep::__vecDelDtor this adjustor: 12
vbi:       class  offset o.vbptr  o.vbte fVtorDisp
          Animal      12       4       4 0

Tuo的記憶體布局

class Tuo       size(20):
        +---
 0      | {vfptr}
 4      | {vbptr}
 8      | tuo
        +---
        +--- (virtual base Animal)
12      | {vfptr}
16      | animal
        +---

Tuo::[email protected]@:
        | &Tuo_meta
        |  0
 0      | &Tuo::h

Tuo::[email protected]:
 0      | -4
 1      | 8 (Tuod(Tuo+4)Animal)

Tuo::[email protected]@:
        | -12
 0      | &Tuo::{dtor}
 1      | &Tuo::f
 2      | &Tuo::g

Tuo::{dtor} this adjustor: 12
Tuo::f this adjustor: 12
Tuo::g this adjustor: 12
Tuo::h this adjustor: 0
Tuo::__delDtor this adjustor: 12
Tuo::__vecDelDtor this adjustor: 12
vbi:       class  offset o.vbptr  o.vbte fVtorDisp
          Animal      12       4       4 0

SheepTuo的記憶體布局，由于它的繼承并非虛繼承，所有自身的類布局放在最後，且沒有vbptr.

class SheepTuo  size(36):
        +---
 0      | +--- (base class Sheep)
 0      | | {vfptr}
 4      | | {vbptr}
 8      | | sheep
        | +---
12      | +--- (base class Tuo)
12      | | {vfptr}
16      | | {vbptr}
20      | | tuo
        | +---
24      | sheep_tuo
        +---
        +--- (virtual base Animal)
28      | {vfptr}
32      | animal
        +---

SheepTuo::[email protected]@:
        | &SheepTuo_meta
        |  0
 0      | &SheepTuo::h

SheepTuo::[email protected]@:
        | -12
 0      | &thunk: this-=12; goto SheepTuo::h

SheepTuo::[email protected]@:
 0      | -4
 1      | 24 (SheepTuod(Sheep+4)Animal)

SheepTuo::[email protected]@:
 0      | -4
 1      | 12 (SheepTuod(Tuo+4)Animal)

SheepTuo::[email protected]@:
        | -28
 0      | &SheepTuo::{dtor}
 1      | &SheepTuo::f
 2      | &SheepTuo::g

SheepTuo::f this adjustor: 28
SheepTuo::g this adjustor: 28
SheepTuo::h this adjustor: 0
SheepTuo::{dtor} this adjustor: 28
SheepTuo::__delDtor this adjustor: 28
SheepTuo::__vecDelDtor this adjustor: 28
vbi:       class  offset o.vbptr  o.vbte fVtorDisp
          Animal      28       4       4 0
Microsoft (R) Incremental Linker Version 14.16.27045.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:main.exe
main.obj

調試視窗看到的布局如下：

虛函數和虛繼承虛繼承帶虛函數的多重繼承的記憶體布局參考

帶虛函數的多重繼承的記憶體布局

#include <iostream>

using namespace std;

class Base1
{
public:  //三個虛函數
    virtual void f() { cout << "Base1::f" << endl; }
    virtual void g() { cout << "Base1::g" << endl; }
    virtual void h() { cout << "Base1::h" << endl; }

    int base1;
};

class Base2
{
public:  //三個虛函數
    virtual void f() { cout << "Base2::f" << endl; }
    virtual void g() { cout << "Base2::g" << endl; }
    virtual void h() { cout << "Base2::h" << endl; }

    int base2;
};

class Base3
{
public:  //三個虛函數
    virtual void f() { cout << "Base3::f" << endl; }
    virtual void g() { cout << "Base3::g" << endl; }
    virtual void h() { cout << "Base3::h" << endl; }

    int base3;
};

class Derive :public Base1, public Base2, public Base3
{
public:
    virtual void f() override { cout << "Derive::f" << endl; } //唯一一個覆寫的子類函數
    virtual void g1() { cout << "Derive::g1" << endl; }
    virtual void h1() { cout << "Derive::h1" << endl; }
    virtual void j1() { cout << "Derive::h1" << endl; }

    int derive;
};

int main()
{
    Derive d;
    cout << "sizeof d: " << sizeof(d) << endl;
    return 0;
}

其記憶體布局如下圖所示：

虛函數和虛繼承虛繼承帶虛函數的多重繼承的記憶體布局參考

紅框框中的部分表明虛函數表的大小，包含結束标志。

D:\Fzz\src\tectCPP\tectCPP>cl /d1reportSingleClassLayoutDerive main.cpp
用于 x86 的 Microsoft (R) C/C++ 優化編譯器 19.16.27045 版
版權所有(C) Microsoft Corporation。保留所有權利。

main.cpp
e:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\VC\Tools\MSVC\14.16.27023\include\xlocale(319): warning C4530: 使用了 C++ 異常處理程式，但未啟用展開語義。請指定 /EHsc

class Derive    size(28):
        +---
 0      | +--- (base class Base1)
 0      | | {vfptr}
 4      | | base1
        | +---
 8      | +--- (base class Base2)
 8      | | {vfptr}
12      | | base2
        | +---
16      | +--- (base class Base3)
16      | | {vfptr}
20      | | base3
        | +---
24      | derive
        +---

Derive::[email protected]@:
        | &Derive_meta
        |  0
 0      | &Derive::f
 1      | &Base1::g
 2      | &Base1::h
 3      | &Derive::g1
 4      | &Derive::h1
 5      | &Derive::j1

Derive::[email protected]@:
        | -8
 0      | &thunk: this-=8; goto Derive::f
 1      | &Base2::g
 2      | &Base2::h

Derive::[email protected]@:
        | -16
 0      | &thunk: this-=16; goto Derive::f
 1      | &Base3::g
 2      | &Base3::h

Derive::f this adjustor: 0
Derive::g1 this adjustor: 0
Derive::h1 this adjustor: 0
Derive::j1 this adjustor: 0
Microsoft (R) Incremental Linker Version 14.16.27045.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:main.exe
main.obj

參考

圖文詳解虛繼承的實作原理（記憶體布局分析）

虛函數和虛繼承虛繼承帶虛函數的多重繼承的記憶體布局參考

虛繼承

帶虛函數的多重繼承的記憶體布局

參考

繼續閱讀

利用虛函數設計統一的公共接口（求體積與表面積）

構造函數、析構函數是否要定義為虛函數？

用C來模拟實作多态？

虛函數組實作虛級聯繼承 - 全開放和（接近）全閉合模式

C++虛函數簡析

函數和虛函數對struct結構體大小的影響

[設計模式][GOF] 24種設計模式概述

虛函數的小秘密

虛繼承記憶體分析

C＋＋虛表與多态

示範Heap Spray(堆噴射)的原理

C++面試題構造函數調用虛函數

構造函數和析構函數是否應該為虛函數構造函數析構函數

c++_基類析構函數為什麼必須是虛函數

C++虛函數簡單的例子

[c++] 使用 typeid 來完成 RTTI