header:

class Session;

using SessionUniquePtr = std::unique_ptr<Session>;
using SessionSharedPtr = std::shared_ptr<Session>;

class Session : public std::enable_shared_from_this<Session>
{
public:

  friend void sessionHandleRead(SessionSharedPtr ptr, const boost::system::error_code& error, bool header);
  void readNext();
  friend void sessionHandleWrite(SessionSharedPtr ptr, const boost::system::error_code& error);
  void writeNext();
};

source:

void Session::writeNext()
{
  if (mSendingPacket || mSendingQueue.empty()) return;

  mSendingPacket = mSendingQueue.pop();
  if (!mSendingPacket) return;

  asio::async_write(mSocket, asio::buffer(mSendingPacket->data(), mSendingPacket->size()),
                    boost::bind(sessionHandleWrite, shared_from_this(), asio::placeholders::error));
                                    ^^^
    // another things,
}

void sessionHandleWrite(SessionSharedPtr ptr, const boost::system::error_code& error)
{
    // do things
}

E:\Info\repos\RedstoneInside\src\session.cxx: In member function 'void redi::Session::writeNext()':
E:\Info\repos\RedstoneInside\src\session.cxx:32:33: error: 'sessionHandleWrite' was not declared in this scope
                     boost::bind(sessionHandleWrite, shared_from_this(), asio::placeholders::error));