unique_and_shared_ptr.cpp

#include <iostream>
#include <memory>
#include <list>
using namespace std;

#define SIZE 4
static int GID = 0;

struct X {
    int id;
    X() : id(GID++) {}
    ~X() { cerr << "~X[" << id << "]" << endl; }
    void foo() const { cout << "X[" << id << "]"; }
};

class uptr : public unique_ptr<X> {
    public:
        uptr() : unique_ptr<X>() {}
        uptr(X * x) : unique_ptr<X>(x) {}
        void foo() const { if (!get()) { cout << "NULL"; return; } (*this)->foo(); }
};

class sptr : public shared_ptr<X> {
    public:
        sptr() : shared_ptr<X>() {}
        sptr(X * x) : shared_ptr<X>(x) {}
        void foo() const { if (!get()) { cout << "NULL"; return; } (*this)->foo(); }
};

class ulst : public list<uptr> {
    private:
        int id;
    public:
        ulst() : list<uptr> () , id(GID++) {}
        void append(X * x) { push_back(uptr(x)); }
        void append(uptr& p) { push_back(uptr(move(p))); }
        void foo() const {
            cout << "List[" << id << "]: [";
            for (auto b = cbegin(), x = b, e = cend(); x!=e; x++) {
                if (x!=b) cout << ", ";
                x->foo();
            };
            cout << "]" << endl;
        }
};

class slst : public list<sptr> {
    private:
        int id;
    public:
        slst() : list<sptr> () , id(GID++) {}
        void append(X * x) { push_back(sptr(x)); }
        void append(sptr& p) { push_back(sptr(p)); }
        void foo() const {
            cout << "List[" << id << "]: [";
            for (auto b = cbegin(), x = b, e = cend(); x!=e; x++) {
                if (x!=b) cout << ", ";
                x->foo();
                cout << "#" << x->use_count();
            };
            cout << "]" << endl;
        }
};

int main()
{
    cout << "List of Unique Pointers" << endl;
    ulst l;
    
    l.foo();
    cout << "Fill list with data." << endl;
    for (int i=0; i<SIZE; i++) l.append(new X());
    l.foo();
    cout << "Append odd members of list once again to the list." << endl;
    {
        auto x = l.begin();
        for (int i=0,n=l.size(); i<n; x++,i++) if (i&1) l.append(*x);
    }
    l.foo();

    cout << "EOF" << endl; GID=0;
    
    cout << "List of Shared Pointers" << endl;
    slst s;
    s.foo();
    cout << "Fill list with data." << endl;
    for (int i=0; i<SIZE; i++) s.append(new X());
    s.foo();
    cout << "Append odd members of list once again to the list." << endl;
    {
        auto x = s.begin();
        for (int i=0,n=s.size(); i<n; x++,i++) if (i&1) s.append(*x);
    }
    s.foo();
    
    cout << "EOF" << endl; GID=0;
    return 0;
}

Output obtained thanks to http://cpp.sh/:

List of Unique Pointers
List[0]: []
Fill list with data.
List[0]: [X[1], X[2], X[3], X[4]]
Append odd members of list once again to the list.
List[0]: [X[1], NULL, X[3], NULL, X[2], X[4]]
EOF
List of Shared Pointers
List[0]: []
Fill list with data.
List[0]: [X[1]#1, X[2]#1, X[3]#1, X[4]#1]
Append odd members of list once again to the list.
List[0]: [X[1]#1, X[2]#2, X[3]#1, X[4]#2, X[2]#2, X[4]#2]
EOF