spooky runtime polymorphism

spooky.cpp

#include <memory>
#include <vector>
#include <iostream>
#include <cstddef>
#include <string>

/// draw
///
/// draw a fundamental type
/// @param[in] t Object to draw
/// @param[in] out Output stream to write to
/// @param[in] position Spacing from left side
template <typename T>
void draw(const T& t, std::ostream& out, std::size_t position) {
  out << std::string(position, ' ') << t << '\n';
}

/// object_t
///
/// Represents a document object
class object_t {
public:
  /// constructor
  ///
  /// @param[in] value Value to store in object_t
  template <typename T>
  object_t(T value)
    : self_(std::make_shared<model_t<T>>(std::move(value))) {}

  /// draw
  ///
  /// Draw the object
  /// @param[in] obj Object to draw
  /// @param[in] out Output stream to draw to
  /// @param[in] position Spacing from left side
  friend void draw(const object_t& obj, std::ostream& out, std::size_t position) {
    obj.self_->draw_(out, position);
  }

private:
  /// concept_t
  ///
  /// Represents a drawable object
  struct concept_t {
    /// draw
    ///
    /// Draw the object
    /// @param[in] out Output stream to draw to
    /// @param[in[ position Spacing from left side
    virtual void draw_(std::ostream& out, std::size_t position) const = 0;

    /// destructor
    virtual ~concept_t() = default;
  };
  
  /// model_t
  ///
  /// Underlying representation of document objects
  template <typename T>
  struct model_t final : concept_t {
    /// constructor
    ///
    /// @param[in] data Data to hold in the document object
    model_t(T data)
      : data_(std::move(data)) {}

    /// draw
    ///
    /// Draw the underlying object
    /// @param[in] out Output stream to draw to
    /// @param[in] position Spacing from left side
    void draw_(std::ostream& out, std::size_t position) const override {
      draw(data_, out, position);
    }

    /// underlying data
    T data_;
  };

  /// holds abstract representation of the object
  std::shared_ptr<const concept_t> self_;
};

using document_t = std::vector<object_t>;

void draw(const document_t& document, std::ostream& out, std::size_t position) {
  out << "<document>\n";
  for (const auto& e : document)
    draw(e, out, position + 2);
  std::cout << "</document>\n";
}

struct my_object_t {
  // blah
};

void draw(const my_object_t& document, std::ostream& out, std::size_t position) {
  out << std::string(position, ' ') << "my_document_t\n";
}

int main() {
  document_t document;
  document.push_back(0);
  document.push_back(1);
  document.push_back("Hello, world!");
  document.push_back(my_object_t());
  draw(document, std::cout, 0);
}