[cpp][stl][lista]problem podczas sortowania obiektów różnego typu w liscie

#include <iostream>
#include <list>
#include <memory>
#include <stdlib.h>    
using namespace std;

class Base {
public:
    int x;
    Base() {
        x = 0;
        cout << "create base" << endl;
    }
    virtual void say() {
        cout << "say base" << this->x << endl;
    }
    virtual ~Base() {
        cout << "delete base" << endl;
    }
    bool operator <( const Base & Obj ) const
    {
        return x < Obj.x;
    }
};


class A
    : public Base
{
public:
    A() {
        x = rand() % 100 + 1;
        cout << "create A" << endl;
    }
    void say() {
        cout << "say A :" << this->x << endl;
    }
    ~A() {
        cout << "delete A" << endl;
    }
};

class B
    : public Base
{
public:
    B() {
        x = rand() % 100;
        cout << "create B" << endl;
    }
    void say() {
        cout << "say B :" << this->x << endl;
    }
    ~B() {
        cout << "delete B" << endl;
    }
};

class C
    : public Base
{
public:
    C() {
        x = rand() % 100;
        cout << "create C" << endl;
    }
    void say() {
        cout << "say C :" << this->x << endl;
    }
    ~C() {
        cout << "delete C" << endl;
    }
};

int main()
{
    A * a = new A();
    B * b = new B();
    C * c = new C();
    cout << "usuwanie-------------------------------------------" << endl;
    delete a; delete b; delete c;
    cout << "listy----------------------------------------------" << endl;
   
    list < unique_ptr < Base >> my_list;
    my_list.push_back( unique_ptr < Base >( new Base() ) );
    my_list.push_back( unique_ptr < Base >( new A() ) );
    my_list.push_back( unique_ptr < Base >( new B() ) );
    my_list.push_back( unique_ptr < Base >( new C() ) );
    cout << "podaj ilosc" << endl;
    int x;
    cin >> x;
    for( int i = 0; i < x; i++ )
    if( rand() % 2 == 0 )
         my_list.push_back( unique_ptr < Base >( new A() ) );
    else if( rand() % 3 == 0 )
         my_list.push_back( unique_ptr < Base >( new B() ) );
    else
         my_list.push_back( unique_ptr < Base >( new C() ) );
   
    cout << endl << "rozmiar listy " << my_list.size() << endl;
   
    for( auto const & i: my_list ) {
        i->say();
    }
    my_list.sort();
    cout << "po sortowaniu" << endl;
    for( auto const & i: my_list ) {
        i->say();
    }
   
    my_list.clear();
   
    cout << endl << "rozmiar listy po usuwaniu " << my_list.size() << endl;
    return 0;
}

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