为了保证两个不同的对象有不同的地址
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#include<iostream>
using namespace std;
class Empty { };
class Derived: Empty { int a; };
int ()
{
cout << sizeof(Derived);
return 0;
}
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Output (with GCC compiler. See this):
以上程序输出不会大于4，一个空的基类不需要用单独一个byte来表示
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#include <iostream>
using namespace std;
class Empty
{};
class Derived1 : public Empty
{};
class Derived2 : virtual public Empty
{};
class Derived3 : public Empty
{
char c;
};
class Derived4 : virtual public Empty
{
char c;
};
class Dummy
{
char c;
};
int ()
{
cout << "sizeof(Empty) " << sizeof(Empty) << endl;
cout << "sizeof(Derived1) " << sizeof(Derived1) << endl;
cout << "sizeof(Derived2) " << sizeof(Derived2) << endl;
cout << "sizeof(Derived3) " << sizeof(Derived3) << endl;
cout << "sizeof(Derived4) " << sizeof(Derived4) << endl;
cout << "sizeof(Dummy) " << sizeof(Dummy) << endl;
return 0;
}
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Output (with GCC compiler. See this):
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sizeof(Empty) 1
sizeof(Derived1) 1
sizeof(Derived2) 4
sizeof(Derived3) 1
sizeof(Derived4) 8：虚继承会创建虚表，指针的大小是4；char的大小是1，但是为了保证字节对齐，需要填充3个字节，总共是8
sizeof(Dummy) 1
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