1、空類的sizeof是1。空類是指沒有成員的類,類中的函數不佔空間,除非是虛函數。
如: class A
{
public:
A(){}
~A(){}
void fun(){}
};
sizeof(A)是1.
注: class A1
{
public:
A1(){}
~A1(){}
void fun(){}
char a[0];
};
sizeof(A1)也是1.(VC6.0下編譯)
2、若類中包含成員,則類對象的大小隻包括其中非靜態成員經過對齊所佔的空間,對齊方式和結構體相同。如:
class A
{
public:
int b;
float c;
char d;
};
sizeof(A)是12.
class A
{
public:
static int a;
int b;
float c;
char d;
};
sizeof(A)是12.
class A
{
public:
static int a;
int b;
float c;
char d;
int add(int x,int y)
{
return x+y;
}
};
sizeof(A)也是12.
3、若類中包含虛函數,則無論有幾個虛函數,sizeof類都等於sizeof(數據成員)的和+sizeof(V表指針,爲4),如:
class Base
{
public:
Base(){cout<<"Base-ctor"<<endl;}
~Base(){cout<<"Base-dtor"<<endl;}
int a;
virtual void f(int) {cout<<"Base::f(int)"<<endl;}
virtual void f(double){cout<<"Base::f(double)"<<endl;}
};
sizeof(Base)爲8.
4、對於子類,它的sizeof是它父類成員(無論成員是public或private),再加上它自己的成員,對齊後的sizeof,如:
class A2
{
public:
int a;
private:
char b;
};
class A3:public A2
{
public:
char b;
short a;
};
sizeof(A3)是8. 但如果A3如下:
class A3:public A2
{
public:
short a;
char b;
};
sizeof(A3)是12.
5、對於子類和父類中都有虛函數的情況,子類的sizeof是它父類成員(無論成員是public或private),再加上它自己的成員,對齊後的sizeof,再加4(虛表指針)。如:
class Base
{
public:
Base(){cout<<"Base-ctor"<<endl;}
~Base(){cout<<"Base-dtor"<<endl;}
int a;
virtual void f(int) {cout<<"Base::f(int)"<<endl;}
virtual void f(double){cout<<"Base::f(double)"<<endl;}
};
class Derived:public Base
{
public:
Derived(){cout<<"Derived-ctor"<<endl;}
int b;
virtual void g(int){cout<<"Derived::g(int)"<<endl;}
};
sizeof(Derived)是12.
6、對於虛繼承的子類,其sizeof的值是其父類成員,加上它自己的成員,以及它自己一個指向父類的指針(大小爲4),對齊後的sizeof。如:
#include <iostream.h>
class a
{
private:
int x;
};
class b: virtual public a
{
private:
int y;
};
class c: virtual public a
{
private:
int z;
};
class d:public b,public c
{
private:
int m;
};
int main(int argc, char* argv[])
{
cout<<sizeof(a)<<endl;
cout<<sizeof(b)<<endl;
cout<<sizeof(c)<<endl;
cout<<sizeof(d)<<endl;
return 0;
}
在VC6.0下調試結果爲
4
12
12
24
sizeof(b)和sizeof(c)相同,都是4+4+4=12。
sizeof(d)是sizeof(b)(爲12)+sizeof(c)(爲12)-b和c相同的部分(a的成員,大小是4)+d自己的成員(大小爲4)=24
7、對於既有虛繼承又有虛函數的子類,其sizeof的值是其父類成員(計算虛表指針大小+4),加上它自己的成員(計算虛表指針大小+4),以及它自己一個指向父類的指針(大小爲4),對齊後的sizeof
class Base
{
public:
Base(){cout<<"Base-ctor"<<endl;}
~Base(){cout<<"Base-dtor"<<endl;}
virtual void f(int) {cout<<"Base::f(int)"<<endl;}
virtual void f(double){cout<<"Base::f(double)"<<endl;}
};
class Derived:virtual public Base
{
public:
Derived(){cout<<"Derived-ctor"<<endl;}
virtual void g(int){cout<<"Derived::g(int)"<<endl;}
};
sizeof(Base)=4
sizeof(Derived)=12 (父類虛表指針大小4+自己虛表指針大小4+子類指向父類的一個指針大小4=12)