1.常见类型实例大小（32位环境下）
空类的实例：1字节，理由：任意实例在内存中都有唯一的地址，为此，编译器给空类的实例加一个字节空间。
enum：4字节。
2.字节对齐（略）
3.同类型不同平台的差异（略）
4.virtual
注意：本次是32位环境下
#include <iostream>
using namespace std;
#pragma pack(4)
class Base
{
public:
virtual void func(){}
};
class Child001 :
public Base
{
public:
int m_a;
char m_b;
};
class Child002
{
public:
int m_a;
char m_b;
public:
virtual void func(){}
};
class Child003
{
public:
int m_a;
char m_b;
public:
virtual void func(){}
virtual void func2(){}
};
class Base002
{
public:
virtual void func(){}
virtual void func1(){}
};
class Child004 :
public Base002
{
public:
int m_a;
char m_b;
public:
virtual void func(){}
virtual void func2(){}
virtual void func3(){}
};
class Child005 :
public Base002
{
public:
char m_a;
public:
virtual void func(){}
virtual void func1(){}
public:
char m_b;
};
class Child006 :
public Base002
{
public:
char m_c;
char m_d;
public:
virtual void func(){}
virtual void func2(){}
};
class Child007
{
public:
virtual void func(){}
};
class Child008 :
public Base002
{
public:
char m_c;
char m_d;
void *
m_e;
public:
virtual void func(){}
virtual void func2(){}
};
class Child009 :
public Base002
{
public:
char m_c;
char m_d;
void *
m_e;
};
class Child010
{
public:
char m_c;
char m_d;
void *
m_e;
};
class Child011
{
public:
char m_c;
char m_d;
};
struct Child012
{
char m_c;
char m_d;
};
class Base003
{
public:
};
class Child013 :
virtual public Base003
{
public:
};
class Base004
{
public:
virtual void func(){}
};
class Child014 :
virtual public Base004
{
public:
};
class Base005
{
};
class Child015 :
public Base005
{
};
class Base006
{
public:
void func(){}
};
class Child016 :
virtual public Base006
{
};
class Base007
{
public:
virtual void func(){}
};
class Base007_2
{
public:
virtual void func(){}
};
class Child017 :
public Base007,
public Base004,
public Base007_2
{};
class Child018 :
virtual public Base007,
virtual public Base004,
virtual public Base007_2
{};
int main(
int,
char**
)
{
cout <<
sizeof(Child001) << endl;
//output 12
cout <<
sizeof(Child002) << endl;
//output 12
cout <<
sizeof(Child003) << endl;
//output 12
cout <<
sizeof(Child004) << endl;
//output 12
cout <<
sizeof(Child005) << endl;
//output 8
cout <<
sizeof(Child006) << endl;
//output 8
cout <<
sizeof(Child007) << endl;
//output 4
cout <<
sizeof(Child008) << endl;
//output 12
cout <<
sizeof(Child009) << endl;
//output 12
cout <<
sizeof(Child010) << endl;
//output 8
cout <<
sizeof(Child011) << endl;
//output 2
cout <<
sizeof(Child012) << endl;
//output 2
cout <<
sizeof(Child013) << endl;
//output 4
cout <<
sizeof(Child014) << endl;
//output 8
cout <<
sizeof(Child015) << endl;
//output 1
cout <<
sizeof(Child016) << endl;
//output 4
cout <<
sizeof(Child017) << endl;
//output 12
cout <<
sizeof(Child018) << endl;
//output 16
system(
"pause");
return 0;
}
View Code
理由：32位环境下，m_a占4字节，m_b的数据占用了1字节，但由于字节对齐，m_b的字段共占4字节（补齐4，m_c和m_d同理），本身存储了一个指向自身虚函数表的指针占用了4字节（该指针不会随着虚函数的增加而增加），从例子中，虚函数表的指针不需要考虑声明顺序的问题。
空类继承空类，子类：1字节
空类虚继承空类，子类：4字节（子类带虚函数表指针）
空类虚继承带方法（非虚函数）的类，子类：4字节（子类带虚函数表指针）
空类虚继承带虚函数的类，子类：8字节（子类和父类带虚函数表指针）
总结：
每继承一个带虚函数表的空基类就累加4字节。
虚继承让派生类（即使是空类）带虚函数表于是产生4字节，而普通继承时派生类不带虚函数则不会有虚函数表，即不会产生4字节。
转载于:https://www.cnblogs.com/xiaobalang/p/7856606.html
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