类的内存布局-sizeof(类对象)
一、简单对象 #include<iostream.h> class A { public: int a; char b; static int c; virtual ~A(){}; };
int A::c=123;
int main() { A asimple; cout<<sizeof(asimple)<<endl; return 0; } //result=4+4+1+3=12即字节对齐(虚指针+非静态数据成员)
二、单继承
#include<iostream.h> class A { public: int a; char b; static int c; virtual ~A(){}; };
int A::c=123;
class B:public A { public: int d; virtual void foob(){}; };
int main() { // A asimple; B bsimple; cout<<sizeof(bsimple)<<endl; return 0; } //result=(4+4+1+3)+4=16即字节对齐(基类size+B自己的非静态数据成员)
三、多继承
#include<iostream.h> class A { public: int a; char b; static int c; virtual ~A(){}; };
int A::c=123;
class B { public: int d; virtual void foob(){}; }; class C:public A,public B { public: int e; };
int main() { A asimple; B bsimple; C csimple; cout<<sizeof(asimple)<<endl; //12 cout<<sizeof(bsimple)<<endl; //8 cout<<sizeof(csimple)<<endl; //12+8+4=24 即字节对齐(基类A大小+基类B大小+C自己的非静态数据成员) return 0; }
四、菱形继承 1.非虚拟继承方式 #include<iostream.h> class A { public: int a; char b; static int c; virtual ~A(){}; virtual void fooa(){}; };
int A::c=123;
class B:public A { public: int d; virtual void foob(){}; }; class C:public A { public: int e; };
class D:public B,public C { public: char f; };
int main() { A asimple; B bsimple; C csimple; D dsimple; cout<<sizeof(asimple)<<endl;//12 cout<<sizeof(bsimple)<<endl;//16 cout<<sizeof(csimple)<<endl;//16 cout<<sizeof(dsimple)<<endl;//16+16+4=36 同多继承方式 return 0; } 2.虚拟继承 (1)B中重写fooa(),但不定义新的虚函数 #include<iostream.h> class A { public: int a; char b; static int c; virtual ~A(){}; virtual void fooa(){}; };
int A::c=123;
class B:virtual public A { public: int d; // virtual void foob(){}; virtual void fooa(){cout<<"fooa in B"<<endl;}; }; class C:virtual public A { public: int e; };
class D:public B,public C { public: char f; };
int main() { A asimple; B bsimple; C csimple; D dsimple; cout<<sizeof(asimple)<<endl;//12 cout<<sizeof(bsimple)<<endl;//20 cout<<sizeof(csimple)<<endl;//20 cout<<sizeof(dsimple)<<endl;//32 return 0; }
(2)B定义新的虚函数 #include<iostream.h> class A { public: int a; char b; static int c; virtual ~A(){}; virtual void fooa(){}; };
int A::c=123;
class B:virtual public A { public: int d; virtual void foob(){cout<<"foob in B"<<endl;}; // virtual void fooa(){cout<<"fooa in B"<<endl;}; }; class C:virtual public A { public: int e; };
class D:public B,public C { public: char f; };
int main() { A asimple; B bsimple; C csimple; D dsimple; cout<<sizeof(asimple)<<endl;//12 cout<<sizeof(bsimple)<<endl;//24 cout<<sizeof(csimple)<<endl;//20 cout<<sizeof(dsimple)<<endl;//36 return 0; }
综合1，2，可见 1.非虚拟继承时，无论派生类是否定义新的虚函数，基类和派生类总是共享一个虚函数表，不需要另加指向虚函数的指针。 2.虚拟继承时，若是派生类只是继承或重写基类中虚函数，则基类和派生类是共享一个虚函数表；若派生类新定义了虚函数，则需要新加一个虚指针指向新的虚函数表。
