概述

虛函式表是C++實作多型的一種方式，
問題:

• 什么情況下C++會使用虛指標和虛函式表？
• 如果子類不新增任何虛函式，也不重寫父類的虛方法，會和父類共用一張虛函式表么？
• 父類的建構式為什么不能正確的呼叫虛函式？

C++虛函式表指標和虛函式表

創建一個Base類

class Base
{
public:
	int a;
	int b;
};

查看Base記憶體布局

1>class Base size(8):
1> +---
1> 0 | a
1> 4 | b
1> +---

為Base類添加一個虛函式

class Base
{
public:
	int a;
	int b;

	virtual void BaseFunc1()
	{
		std::cout << "Call BaseFunc1 From Base" << std::endl;
	};
};

此時再查看Base類的記憶體布局

1>class Base size(12):
1> +---
1> 0 | {vfptr}
1> 4 | a
1> 8 | b
1> +---
1>Base::$vftable@:
1> | &Base_meta
1> | 0
1> 0 | &Base::BaseFunc1
1>Base::BaseFunc1 this adjustor: 0

Base類含有虛函式時,.rodata只讀資料區會生成一個虛函式表，Base類會生成一個指向該虛函式表的指標成員變數，虛函式表存放.text代碼區函式的地址，

再為Base添加一個虛函式

class Base
{
public:
	int a;
	int b;

	virtual void BaseFunc1()
	{
		std::cout << "Call BaseFunc1 From Base" << std::endl;
	};
	
	virtual void BaseFunc2()
	{
		std::cout << "Call BaseFunc2 From Base" << std::endl;
	}
};

查看Base類的記憶體分布

1>class Base size(12):
1> +---
1> 0 | {vfptr}
1> 4 | a
1> 8 | b
1> +---
1>Base::$vftable@:
1> | &Base_meta
1> | 0
1> 0 | &Base::BaseFunc1
1> 1 | &Base::BaseFunc2
1>Base::BaseFunc1 this adjustor: 0
1>Base::BaseFunc2 this adjustor: 0

Base類的虛函式表增加了一個新函式地址，

C++ 虛函式表和多型

為Base創建一個派生類Devire

class Derive : public Base
{
	//
};

查看Derive類的記憶體分布

1>class Derive size(12):
1> +---
1> 0 | +--- (base class Base)
1> 0 | | {vfptr}
1> 4 | | a
1> 8 | | b
1> | +---
1> +---
1>Derive::$vftable@:
1> | &Derive_meta
1> | 0
1> 0 | &Base::BaseFunc1
1> 1 | &Base::BaseFunc2

虛函式表的內容和父類Base一樣

查看Base和Derive的虛函式表地址

Base和Derive并非公用一張虛函式表，

Derive重寫父類Base的方法

class Derive : public Base
{
public:
	virtual void BaseFunc1() override
	{
		std::cout << "Call BaseFunc1 From Derive" << std::endl;
	}
};

查看Derive類的記憶體分布

1>class Derive size(12):
1> +---
1> 0 | +--- (base class Base)
1> 0 | | {vfptr}
1> 4 | | a
1> 8 | | b
1> | +---
1> +---
1>Derive::$vftable@:
1> | &Derive_meta
1> | 0
1> 0 | &Derive::BaseFunc1
1> 1 | &Base::BaseFunc2
1>Derive::BaseFunc1 this adjustor: 0

此時虛函式表的0元素被替換成了Derive::BaseFunc1的地址，

為Derive添加一個新的虛函式

class Derive : public Base
{
public:
	virtual void BaseFunc1() override
	{
		std::cout << "Call BaseFunc1 From Derive" << std::endl;
	}

	virtual void DeriveFunc1()
	{
		std::cout << "Call DeriveFunc1" << std::endl;
	}
};

再查看Derive類的記憶體分布

1>class Derive size(12):
1> +---
1> 0 | +--- (base class Base)
1> 0 | | {vfptr}
1> 4 | | a
1> 8 | | b
1> | +---
1> +---
1>Derive::$vftable@:
1> | &Derive_meta
1> | 0
1> 0 | &Derive::BaseFunc1
1> 1 | &Base::BaseFunc2
1> 2 | &Derive::DeriveFunc1
1>Derive::BaseFunc1 this adjustor: 0
1>Derive::DeriveFunc1 this adjustor: 0

Derive的虛函式表添加了一個新的函式地址，

讓父類Base在建構式中呼叫虛函式BaseFunc1，

class Base
{
public:
	Base()
	{
		BaseFunc1()；
	}
	int a;
	int b;

	virtual void BaseFunc1()
	{
		std::cout << "Call BaseFunc1 From Base" << std::endl;
	};

	virtual void BaseFunc2()
	{
		std::cout << "Call BaseFunc2 From Base" << std::endl;
	}
};

class Derive : public Base
{
public:
	virtual void BaseFunc1() override
	{
		std::cout << "Call BaseFunc1 From Derive" << std::endl;
	}

	virtual void DeriveFunc1()
	{
		std::cout << "Call DeriveFunc1" << std::endl;
	}
};

int main()
{
	Derive d;
	return 0;
}

輸出

Call BaseFunc1 From Base

虛函式的呼叫是錯誤的，

查看Derive和Base的建構式的匯編代碼

Base 建構式匯編代碼

...
00641F4D mov dword ptr [eax],offset Base::`vftable' (0649B34h)
{
BaseFunc1();
00641F53 mov ecx,dword ptr [this]
00641F56 call Base::BaseFunc1 (0641488h)
}
00641F5B mov eax,dword ptr [this]
...

Derive 建構式的匯編代碼

...
0064220A mov ecx,dword ptr [this]
0064220D call Base::Base (06412B2h)
{
00642212 mov eax,dword ptr [this]
00642215 mov dword ptr [eax],offset Derive::`vftable' (0649B40h)
//
}
...

觀察匯編代碼可知，構造Devire類的物件時，當呼叫父類Base的建構式時，此時虛指標指向的虛函式表是父類Base的，只有呼叫Derive自己的建構式時，虛指標被賦值為Derive的虛函式表，所以父類的建構式不能正確的呼叫虛函式，

標籤：C++

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