c++ - 存储在 ptr_vector 中的派生类未被破坏

Question

试图找到使用 ptr_vector 存储、访问和释放对象的最佳方法，尤其是当存储的对象是从其他对象继承时(ptr_vector 不应该有对象切片的任何问题)。但是当运行下面的程序时，令人惊讶的是派生类没有被破坏。有人知道为什么吗？

#include <boost/ptr_container/ptr_vector.hpp>
#include <iostream>
#include <boost/ptr_container/ptr_map.hpp>
#include <boost/foreach.hpp>
using namespace std;

class A
{
 public:
 int id;
 A() {cout<<"Constructed A()"<<endl;}
 A(int i):id(i) {cout<<"Constructed A"<<i<<endl;}
 ~A() {cout<<"* Destructed A"<<id<<endl;}
};
class B:public A
{
 public:
 int i;
 B() {cout<<"Constructed B"<<endl;}
 B(int ii):i(ii) {id=ii;cout<<"Constructed B"<<i<<endl;}
 ~B() {cout<<"* Destructed B"<<i<<endl;}
};

class zoo
{
 boost::ptr_vector<A> the_animals;
public:
 void addAnimal(A* a) {the_animals.push_back( a );}
 void removeAnimal(int id) {the_animals.release(the_animals.begin()+id); }
 void removeOwnership(int id) {the_animals.release(the_animals.begin()+id).release();}
};

int main()
{
 zoo z;
 z.addAnimal( new B(0) );
 //delete abc;z.addAnimal(abc);//doing this will cause heap corruption
 B* lion=new B(1);
 z.addAnimal(lion);
 z.removeOwnership(1);
        delete lion;
 z.removeAnimal(0);
}//main

程序的输出是:

Constructed A()
Constructed B0
Constructed A()
Constructed B1
* Destructed B1
* Destructed A1
* Destructed A0

为什么 B0 没有被销毁？对象被切片了吗？

Answer 1

基类的析构函数不是虚函数:

 ~A() {cout<<"* Destructed A"<<id<<endl;}

应该是:

 virtual ~A() {cout<<"* Destructed A"<<id<<endl;}

为什么？参见 When should your destructor be virtual?