sizeof

c++虚继承

by 
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#include <iostream>
 
using namespace std;
 
class Base {
public:
	virtual ~Base() {}
	virtual void func() = 0;
	int base_;
};
 
class SubLeft : virtual public Base {
public:
	int subLeft_;
};
 
class SubRight : virtual public Base {
public:
	int subRight_;
};
 
class Diamond : public SubLeft, public SubRight {
public:
	void func() {}
	int diamond_;
};
 
int main()
{
	Diamond diamond;
	cout << "sizeof Base: " << sizeof(Base) << endl;
	cout << "sizeof SubLeft: " << sizeof(SubLeft) << endl;
	cout << "sizeof Diamond: " << sizeof(Diamond) << endl;
	cout << "base_ offset: " << (size_t)&diamond.base_ - (size_t)&diamond << endl;
	cout << "subLeft_ offset: " << (size_t)&diamond.subLeft_ - (size_t)&diamond << endl;
	return 1;
}

编译环境是vs2019 x64，项目属性 => C/C++ => Command Line中增加

1

/d1reportSingleClassLayoutDiamond

编译时输出

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1>class SubLeft	size(32):
1>	+---
1> 0	| {vbptr}
1> 8	| subLeft_
1>  	| <alignment member> (size=4)
1>	+---
1>	+--- (virtual base Base)
1>16	| {vfptr}
1>24	| base_
1>  	| <alignment member> (size=4)
1>	+---
1>class Diamond	size(56):
1>	+---
1> 0	| +--- (base class SubLeft)
1> 0	| | {vbptr}
1> 8	| | subLeft_
1>  	| | <alignment member> (size=4)
1>	| +---
1>16	| +--- (base class SubRight)
1>16	| | {vbptr}
1>24	| | subRight_
1>  	| | <alignment member> (size=4)
1>	| +---
1>32	| diamond_
1>  	| <alignment member> (size=4)
1>	+---
1>	+--- (virtual base Base)
1>40	| {vfptr}
1>48	| base_
1>  	| <alignment member> (size=4)
1>	+---
1>Diamond::$vbtable@SubLeft@:
1> 0	| 0
1> 1	| 40 (Diamondd(SubLeft+0)Base)
1>Diamond::$vbtable@SubRight@:
1> 0	| 0
1> 1	| 24 (Diamondd(SubRight+0)Base)
1>Diamond::$vftable@:
1>	| -40
1> 0	| &Diamond::{dtor}
1> 1	| &Diamond::func
1>Diamond::func this adjustor: 40
1>Diamond::{dtor} this adjustor: 40
1>Diamond::__delDtor this adjustor: 40
1>Diamond::__vecDelDtor this adjustor: 40
1>vbi:	   class  offset o.vbptr  o.vbte fVtorDisp
1>            Base      40       0       4 0

运行输出

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sizeof Base: 16
sizeof SubLeft: 32
sizeof Diamond: 56
base_ offset: 48
subLeft_ offset: 8

refer to: https://blog.csdn.net/oracleot/article/details/5123657