为什么这个类对象有40byte? 仨成员变量加一起才9，对齐也才12

#include <iostream>
#include <string>
using namespace std;

class Teacher
{
public:
    Teacher(int id, string nam, char se) : num(id), name(nam), sex(se) {}

protected:       //*派生类能访问的private  （派生不能直接访问private
    int num;     //?  4 byte
    string name; //?  4 byte
    char sex;    //?  1 byte
};

int main()
{
    int num;
    string name;
    char sex;
    int year, month, day;
    cin >> num >> name >> sex;

    Teacher a(num, name, sex);

    cout << sizeof(a) << endl;
    return 0;
}