c++ - Dynamic casting for unique_ptr

Question

As it was the case in Boost, C++11 provides some functions for casting shared_ptr:

std::static_pointer_cast
std::dynamic_pointer_cast
std::const_pointer_cast

I am wondering, however, why there are no equivalents functions for unique_ptr.

Consider the following simple example:

class A { virtual ~A(); ... }
class B : public A { ... }

unique_ptr<A> pA(new B(...));

unique_ptr<A> qA = std::move(pA); // This is legal since there is no casting
unique_ptr<B> pB = std::move(pA); // This is not legal

// I would like to do something like:
// (Of course, it is not valid, but that would be the idea)
unique_ptr<B> pB = std::move(std::dynamic_pointer_cast<B>(pA));

Is there any reason why this usage pattern is discouraged, and thus, equivalent functions to the ones present in shared_ptr are not provided for unique_ptr?

Answer 1

In addition to Mark Ransom's answer, a unique_ptr<X, D> might not even store an X*.

If the deleter defines the type D::pointer then that's what is stored, and that might not be a real pointer, it only needs to meet the NullablePointer requirements and (if unique_ptr<X,D>::get() is called) have an operator* that returns X&, but it isn't required to support casting to other types.

unique_ptr is quite flexible and doesn't necessarily behave very much like a built-in pointer type.

As requested, here is an example where the stored type is not a pointer, and therefore casting is not possible. It's a bit contrived, but wraps a made-up database API (defined as a C-style API) in a C++ RAII-style API. The OpaqueDbHandle type meets the NullablePointer requirements, but only stores an integer, which is used as a key to lookup the actual DB connection via some implementation-defined mapping. I'm not showing this as an example of great design, just as an example of using unique_ptr to manage a non-copyable, movable resource which is not a dynamically-allocated pointer, where the "deleter" doesn't just call a destructor and deallocate memory when the unique_ptr goes out of scope.

#include <memory>

// native database API
extern "C"
{
  struct Db;
  int db_query(Db*, const char*);
  Db* db_connect();
  void db_disconnect(Db*);
}

// wrapper API
class OpaqueDbHandle
{
public:
  explicit OpaqueDbHandle(int id) : id(id) { }

  OpaqueDbHandle(std::nullptr_t) { }
  OpaqueDbHandle() = default;
  OpaqueDbHandle(const OpaqueDbHandle&) = default;

  OpaqueDbHandle& operator=(const OpaqueDbHandle&) = default;
  OpaqueDbHandle& operator=(std::nullptr_t) { id = -1; return *this; }

  Db& operator*() const;

  explicit operator bool() const { return id > 0; }

  friend bool operator==(const OpaqueDbHandle& l, const OpaqueDbHandle& r)
  { return l.id == r.id; }

private:
  friend class DbDeleter;
  int id = -1;
};

inline bool operator!=(const OpaqueDbHandle& l, const OpaqueDbHandle& r)
{ return !(l == r); }

struct DbDeleter
{
  typedef OpaqueDbHandle pointer;

  void operator()(pointer p) const;
};

typedef std::unique_ptr<Db, DbDeleter> safe_db_handle;

safe_db_handle safe_connect();

int main()
{
  auto db_handle = safe_connect();
  (void) db_query(&*db_handle, "SHOW TABLES");
}


// defined in some shared library

namespace {
  std::map<int, Db*> connections;      // all active DB connections
  std::list<int> unused_connections;   // currently unused ones
  int next_id = 0;
  const unsigned cache_unused_threshold = 10;
}

Db& OpaqueDbHandle::operator*() const
{
   return connections[id];
}

safe_db_handle safe_connect()
{
  int id;
  if (!unused_connections.empty())
  {
    id = unused_connections.back();
    unused_connections.pop_back();
  }
  else
  {
    id = next_id++;
    connections[id] = db_connect();
  }
  return safe_db_handle( OpaqueDbHandle(id) );
}

void DbDeleter::operator()(DbDeleter::pointer p) const
{
  if (unused_connections.size() >= cache_unused_threshold)
  {
    db_disconnect(&*p);
    connections.erase(p.id);
  }
  else
    unused_connections.push_back(p.id);
}