main.cpp

#include <cstdio>
#include <memory>
#include <string>
#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

template<typename T>
void draw(const T &x, ostream &out, size_t position)
{
    out << string(position, ' ') << x << endl;
}


class object_t
{
public:
    template<typename T>
    object_t(T x) : self_(make_unique<model<T>>(move(x)))
    {}

    object_t(const object_t &x) : self_(x.self_->copy_())
    {}

    object_t(object_t &&) noexcept = default;

    object_t &operator=(const object_t &x)
    { return *this = object_t(x); }

    object_t &operator=(object_t &&) noexcept = default;

    friend void draw(const object_t &x, ostream &out, size_t position)
    {
        x.self_->draw_(out, position);
    }

private:

    struct concept_t {
        ~concept_t() = default;
        virtual unique_ptr<concept_t> copy_() const = 0;
        virtual void draw_(ostream &, size_t) const = 0;
    };

    template<typename T>
    struct model final : concept_t {
        model(T x) : data_(move(x)) {}
        unique_ptr<concept_t> copy_() const override
        {
            return make_unique<model>(*this);
        }
        void draw_(ostream &out, size_t position) const override
        {
            draw(data_, out, position);
        }

        T data_;
    };

    unique_ptr<concept_t> self_;
};

using document_t = vector<object_t>;

template<>
void draw(const document_t &x, ostream &out, size_t position)
{
    out << string(position, ' ') << "<document>" << endl;
    for(const auto &e : x) {
        draw(e, out, position + 2);
    }
    out << string(position, ' ') << "</document>" << endl;
}

int main(int argc, char *argv[])
{
    document_t document;

    document.emplace_back(0);
    document.emplace_back(1);
    document.emplace_back(string("a string"));
    document.emplace_back(3);

    draw(document, cout, 0);

    reverse(document.begin(), document.end());

    draw(document, cout, 0);

    return 0;
}