概述

C++的多態(tài)在不同環(huán)境下實現(xiàn)方式可能不一樣，虛函數(shù)表是C++實現(xiàn)多態(tài)的一種方式。

問題:

• 什么情況下C++會使用虛指針和虛函數(shù)表？
• 如果子類不新增任何虛函數(shù)，也不重寫父類的虛方法，會和父類共用一張?zhí)摵瘮?shù)表么？
• 父類的構(gòu)造函數(shù)為什么不能正確的調(diào)用虛函數(shù)？

C++虛函數(shù)表指針和虛函數(shù)表

創(chuàng)建一個Base類

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

查看Base內(nèi)存布局

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

為Base類添加一個虛函數(shù)

class Base
{
public:
	int a;
	int b;

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

此時再查看Base類的內(nèi)存布局

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類含有虛函數(shù)時,.rodata只讀數(shù)據(jù)區(qū)會生成一個虛函數(shù)表，Base類會生成一個指向該虛函數(shù)表的指針成員變量。虛函數(shù)表存放.text代碼區(qū)函數(shù)的地址。

再為Base添加一個虛函數(shù)

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類的內(nèi)存分布

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類的虛函數(shù)表增加了一個新函數(shù)地址。

C++ 虛函數(shù)表和多態(tài)

為Base創(chuàng)建一個派生類Devire

class Derive : public Base
{
	//
};

查看Derive類的內(nèi)存分布

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

虛函數(shù)表的內(nèi)容和父類Base一樣

查看Base和Derive的虛函數(shù)表地址

Base和Derive并非公用一張?zhí)摵瘮?shù)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類的內(nèi)存分布

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

此時虛函數(shù)表的0元素被替換成了Derive::BaseFunc1的地址。

為Derive添加一個新的虛函數(shù)

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類的內(nèi)存分布

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的虛函數(shù)表添加了一個新的函數(shù)地址。

讓父類Base在構(gòu)造函數(shù)中調(diào)用虛函數(shù)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

虛函數(shù)的調(diào)用是錯誤的。

查看Derive和Base的構(gòu)造函數(shù)的匯編代碼

Base 構(gòu)造函數(shù)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 構(gòu)造函數(shù)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(gòu)造Devire類的對象時，當(dāng)調(diào)用父類Base的構(gòu)造函數(shù)時，此時虛指針指向的虛函數(shù)表是父類Base的，只有調(diào)用Derive自己的構(gòu)造函數(shù)時，虛指針被賦值為Derive的虛函數(shù)表，所以父類的構(gòu)造函數(shù)不能正確的調(diào)用虛函數(shù)。

以上就是C++學(xué)習(xí)之虛函數(shù)表與多態(tài)詳解的詳細(xì)內(nèi)容，更多關(guān)于C++虛函數(shù)表 多態(tài)的資料請關(guān)注腳本之家其它相關(guān)文章！

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