关于sizeof

  1 #include <QCoreApplication>
  2 #include <QTextStream>
  3 #include <stdio.h>
  4 QTextStream cin(stdin, QIODevice::ReadOnly);
  5 QTextStream cout(stdout, QIODevice::WriteOnly);
  6 class CBase
  7 {
  8  /*类的实例化就是在内存中分配一块地址，空类也会被实例化，
  9    所以编译器给空类隐含的添加一个字节,这样空类实例化后就有了一个独一无二的地址了，
 10    所以空类的sizeof为1
 11    */
 12 };
 13 class CBase2
 14 {
 15     int a;//内部变量的大小之和
 16     char* p;
 17 };
 18 class CBase3
 19 {
 20 public:
 21     CBase3(void);
 22     virtual ~CBase3(void);//c++类中有虚函数的时候有一个指向虚函数的指针vptr
 23 private:
 24     int a;
 25     char *p;
 26 };
 27 class CChild: public CBase2
 28 {
 29 public:
 30     CChild(void);
 31     ~CChild(void);
 32 private:
 33     int b;//继承类的sizeof是
 34 };
 35 //虚拟继承（相当于添加了一个接口)
 36 //virtual虚继承让长度增加了4，其实是多了一个指针，它指向一个关于虚基类偏移量的数组，偏移量是关于虚基类数据成员的偏移量
 37 //注意，这个指针不是虚函数表指针
 38 class COneMember
 39 {
 40 public:
 41     COneMember(int iValue=0){m_iOne = iValue;};
 42 private:
 43     int m_iOne;
 44 };
 45 class CTwoMember : virtual public COneMember
 46 {
 47 private:
 48     int m_iTwo;
 49 };
 50 //关于闭合继承
 51 class A
 52 {
 53 public:
 54     A(int iValue=1){m_iA = iValue;};
 55 private:
 56     int m_iA;
 57 };
 58 class B :virtual public A
 59 {
 60 public:
 61     B(int iValue=2){m_iB = iValue;};
 62 private:
 63     int m_iB;
 64 };
 65 class C : virtual public A
 66 {
 67 public:
 68     C(int iValue=3){m_iC = iValue;};
 69 private:
 70     int m_iC;
 71 };
 72 class Complex : public B, public C
 73 {
 74 public:
 75     Complex(int iValue=4){m_iComplex = iValue;};
 76 private:
 77     int m_iComplex;
 78 };
 79 //虚空类为4，非虚空类为1
 80 class EVirtual
 81 {
 82 public:
 83     EVirtual(){printf("Construct/n");}
 84     virtual ~EVirtual(){ printf("Destruct/n");}
 85 };
 86 //静态成员不会占用类的大小
 87 class CStaticNull
 88 {
 89 public:
 90     CStaticNull(){};
 91     static void Foo(){printf("Foo/n");};
 92     static int m_iValue;
 93 };
 94 //虚函数:子类有新的虚函数，虚表还是只有一张
 95 class CVirtualNull
 96 {
 97 public:
 98     CVirtualNull();
 99     virtual ~CVirtualNull();
100     virtual Foo();
101 };
102 class CVirtualDerived : public CVirtualNull
103 {
104 public:
105     virtual FooDerived();
106 };
107 int main(int argc, char *argv[])
108 {
109     QCoreApplication a(argc, argv);
110     cout<<"sizeof(CBase)="<<sizeof(CBase)<<endl;
111     cout<<"sizeof(CBase2)="<<sizeof(CBase2)<<endl;
112     cout<<"sizeof(CBase3)="<<sizeof(CBase3)<<endl;
113     cout<<"sizeof(CChild)="<<sizeof(CChild)<<endl;
114     cout<<"sizeof(CTwoMember)="<<sizeof(CTwoMember)<<endl;//12
115     cout<<"sizeof(Complex)="<<sizeof(Complex)<<endl;
116     cout<<"sizeof(EVirtual)="<<sizeof(EVirtual)<<endl;
117     cout<<"sizeof(CStaticNull)="<<sizeof(CStaticNull)<<endl;
118     cout<<"sizeof(CVirtualDerived)="<<sizeof(CVirtualDerived)<<endl;
119     return a.exec();
120 }