class error_category
{
protected:
  error_category() = default;

public:
  virtual const char*
  name() const = 0;
};

const error_category&
get_category();

static const error_category& category = get_category();

class error_code
{
public:
  error_code() : _c(&category) { }

  const error_category& cat() const { return *_c; }

private:
  const error_category* _c;
};