Representation and basic construction of a generalized union type.

From Mechanism to Method
Valued Conversions

Kevlin Henney
Listing 1. Representation and basic construction of a generalized union type.


class any
{
public:
  any()
    : content(0)
  {
  }
  ~any()
  {
      delete content;
  }
  const std::type_info &type_info() const
  {
      return content
        ? content->type_info()
        : typeid(void);
  }
  ...
private:
  class placeholder
  {
  public:
    virtual ~placeholder()
    {
    }
    virtual const std::type_info &
      type_info() const = 0;

    virtual placeholder *clone() const = 0;
  };
  template<typename value_type>
  class holder : public placeholder
  {
  public:
    holder(const value_type &value)
      : held(value)
    {
    }
    virtual const std::type_info &type_info() const
    {
        return typeid(value_type);
    }
    virtual placeholder *clone() const
    {
        return new holder(held);
    }
    const value_type held;
  };
  placeholder *content;
};

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