为什么派生类可以调用基类构造函数两次？

Question

I am learning about multiple inheritance and the diamond problem, but I'm confused as to why a base class constructor can be called twice without a compiler error in the first place.

Example from https://www.geeksforgeeks.org/multiple-inheritance-in-c/

#include<iostream> 
using namespace std; 

class Person { 
   // Data members of person  
public: 
    Person(int x)  { cout << "Person::Person(int ) called" << endl;   } 
    int y;                                                               \\ <- I added this
}; 

class Faculty : public Person { 
   // data members of Faculty 
public: 
    Faculty(int x):Person(x)   { 
       cout<<"Faculty::Faculty(int ) called"<< endl; 
    } 
}; 

class Student : public Person { 
   // data members of Student 
public: 
    Student(int x):Person(x) { 
        cout<<"Student::Student(int ) called"<< endl; 
    } 
}; 

class TA : public Faculty, public Student  { 
public: 
    TA(int x):Student(x), Faculty(x)   { 
        cout<<"TA::TA(int ) called"<< endl; 
    } 
}; 

int main()  { 
    TA ta1(30); 
} 

Output:

Person::Person(int ) called
Faculty::Faculty(int ) called
Person::Person(int ) called
Student::Student(int ) called
TA::TA(int ) called

Since the derived class TA calls the Person constructor twice, doesn't that mean TA would have two copies of data members that have the same name, for example two instances of int y ?

Answer 1

Drawing from link is misleading, it is not a diamond, but more a Y:

 ----------    ----------
|  Person  |  |  Person  |
 ----------    ----------
      ^             ^
      |             |
 ----------    ----------
|  Student |  |  Faculty |
 ----------    ----------
      ^             ^
      |             |
       \-----   ----/
             \ /
              |
         ----------
        |    TA    |
         ----------

And yes you have two copies of member of Person ( Student::y and Faculty::y ).

With virtual inheritance, you have a diamond with only one unique Person .

Answer 2

Instead of printing the integer argument, try printing the address of the member variable instead. Then try it with virtual inheritance, it should give a better idea what's going on.

std::cout << &y << std::endl;

Answer 3

Try investigating the size of the TA object you instantiate. What does the size tell you? What size would you expect for the case of a single copy of Person? For two copies?

Try actually accessing the y attribute and see if you get a compiler error!