C++虚函数与继承-优快云博客

本文探讨了C++中虚函数的工作原理及不同类型的继承如何影响类的内存占用大小。通过多个实例展示了虚继承和多重继承对类大小的影响。

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#include <iostream> 
using namespace std; 
class A   
{   
};  

class B   
{
	char ch;   
	virtual void func0()  {  } 
}; 

class C  
{
	char ch1;
	char ch2;
	virtual void func()  {  }  
	virtual void func1()  {  } 
};

class D: public A, public C
{   
	int d;   
	virtual void func()  {  } 
	virtual void func1()  {  }
};   

class E: public B, public C
{   
	int e;   
	virtual void func0()  {  } 
	virtual void func1()  {  }
};

int main(void)
{
	cout<<"A="<<sizeof(A)<<endl;    //result=1
	cout<<"B="<<sizeof(B)<<endl;    //result=8    
	cout<<"C="<<sizeof(C)<<endl;    //result=8
	cout<<"D="<<sizeof(D)<<endl;    //result=12
	cout<<"E="<<sizeof(E)<<endl;    //result=20
	getchar();
	return 0;
}

结果：

DEV C++:

VC60:

#include <iostream> 
using namespace std; 
class CommonBase
{
	int co;
};

class Base1: virtual public CommonBase
{
public:
	virtual void print1() {  }
	virtual void print2() {  }
private:
	int b1;
};

class Base2: virtual public CommonBase
{
public:
	virtual void dump1() {  }
	virtual void dump2() {  }
private:
	int b2;
};

class Derived: public Base1, public Base2//有两个带有虚函数的基类，虚指针所占大小为4*2(是一个vptr还是2个vptr呢？？？？) 
{
public:
	void print2() {  }
	void dump2() {  }
private:
	int d;
};
int main()
{
    cout<<sizeof(Base1)<<endl;//12
    cout<<sizeof(Base2)<<endl;//12
    cout<<sizeof(Derived)<<endl;//24 如果Base1和Base2不是虚继承，则Common Base会补被当做两份不同的拷贝。从而sizeof(Derived)变成28. 
    getchar();
    return 0;
}

结果：

devc++:

VC60:

#include <iostream> 
using namespace std; 
class A
{
public:
	virtual void aa() {  }
	virtual void aa2() {  }
private:
	char ch[3];
};

class B: virtual public A
{
public:
	virtual void bb() {  }
	virtual void bb2() {  }
};

int main(void)
{
	cout<<"A's size is "<<sizeof(A)<<endl;//8
	cout<<"B's size is "<<sizeof(B)<<endl;//12
	getchar();
	return 0;
}

VC60:

参考：

C++中虚函数工作原理和(虚)继承类的内存占用大小计算

C++ 虚函数表解析

深入理解C++的动态绑定和静态绑定

C++对象大小讨论

对比补充例子：

#include<iostream>
using namespace std;
class B
{
      public:
             virtual void fm()
             {}
};
class D:public B
{
      public:
             virtual void fm()
             {}
};
int main()
{
    cout<<sizeof(B)<<endl;
    cout<<sizeof(D)<<endl;
    getchar();
}

DevC++下：

VC60下：

#include<iostream>
using namespace std;
class B
{
      public:
             virtual void fm()
             {}
};
class D:public B
{
      public:
             virtual void ft()
             {}
};
int main()
{
    cout<<sizeof(B)<<endl;
    cout<<sizeof(D)<<endl;
    getchar();
}

DEVC++:

VC60:

#include<iostream>
using namespace std;
class B
{
      public:
             virtual void fm()
             {}
};
class D:virtual public B
{
      public:
             virtual void fm()
             {}
};
int main()
{
    cout<<sizeof(B)<<endl;
    cout<<sizeof(D)<<endl;
    getchar();
}

DEVC++：

VC60:

#include<iostream>
using namespace std;
class B
{
      public:
             virtual void fm()
             {}
};
class D:virtual public B
{
      public:
             virtual void ft()
             {}
};
int main()
{
    cout<<sizeof(B)<<endl;
    cout<<sizeof(D)<<endl;
    getchar();
}

DEVC++：