Base类只是一个指针，所以Base大小是4个字节。
但Base类中添加一个int a,Base的大小变成8字节。
#include <iostream>
#include <complex>
using namespace std;
class Base1
{
public:
Base1(){cout << "Base1" << endl;}
~Base1(){cout << "~Base1" << endl;}
virtual void f(int){cout << "Base1 f(int)" << endl;}
virtual void f(double){cout << "Base1 f(double)" << endl;}
virtual void g(int i = 20){cout << "Base1 f(int)" << endl;}
virtual void g2(int i = 10){cout << "Base1 f(int)" << endl;}
};
class Base2
{
public:
Base2(){cout << "Base2" << endl;}
~Base2(){cout << "~Base2" << endl;}
virtual void f(int){cout << "Base2 f(int)" << endl;}
virtual void f(double){cout << "Base2 f(double)" << endl;}
virtual void g(int i = 20){cout << "Base2 f(int)" << endl;}
virtual void g2(int i = 10){cout << "Base2 f(int)" << endl;}
/* 4 字节 */
int a;
};
class Base3
{
public:
Base3(){ int a;int b;cout << "Base3" << endl;}
~Base3(){cout << "~Base3" << endl;}
virtual void f(int){cout << "Base3 f(int)" << endl;}
virtual void f(double){cout << "Base3 f(double)" << endl;}
virtual void g(int i = 20){cout << "Base3 f(int)" << endl;}
virtual void g2(int i = 10){cout << "Base3 f(int)" << endl;}
/* 4 字节 */
int a;
/* 4 字节 */
int b;
};
int
main()
{
Base1 a;
/* 结果 4 字节 */
cout << sizeof(Base1) << endl;
Base2 b;
/* 结果 8 字节 */
cout << sizeof(Base2) << endl;
Base3 c;
/* 结果 12 字节 */
cout << sizeof(Base3) << endl;
}
/*
[root@localhost test_class]# g++ class.cpp
[root@localhost test_class]# ./a.out
Base1
4
Base2
8
Base3
12
~Base3
~Base2
~Base1
*/