为什么添加虚拟方法会增加C ++中的类大小？

Question

#include <iostream>
using namespace std;

class a {
  virtual int foo() {
    return 0;
  }
};

class b {
  int foo() {
    return 0;
  }
};

int main() {
  cout << sizeof(b) << endl;
  cout << sizeof(a) << endl;
}

Output (with g++ 4.9, -O3):

1
8

I assume the increase in size is due to adding a vpointer. But I thought the compiler would see that a is not actually deriving or being derived from anything, hence there is no need to add the vpointer?

Answer 1

The vpointer is needed because the compiler cannot guarantee an external (eg shared) library does not use a derived type. The existence-of-derived-class resolution happens at runtime.

Answer 2

Run-time type information. Any polymorphic class creates extra meta-data in the program to make things like typeof and dynamic_cast work. This is in addition to the virtual function table.