Question

Was trying to find the best way to use ptr_vector to store, access and release objects, especially when the stored object is inherited from other (ptr_vector should not have any issues with object slicing).
But when running the below program, surprisingly the derived class isn’t being destructed. Anyone know why?

#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

The output for the program is:

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

Why isn’t B0 being destructed? Did the object get sliced?

Answer 1

The destructor of the base class is not virtual:

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

Should be:

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

Why ? See When should your destructor be virtual?