library.hpp

#include <iostream>
#include <vector>

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 {
        virtual ~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>;

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;
}