[Boost-users] shared_ptr problems

3 Nov 2005

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Hi!

I'm trying to create a wrapper around mysql but I have a problem with
shared_ptr. Look at the following code:

#include <boost/shared_ptr.hpp>
#include <iostream>
#include <map>

struct MYSQL {};
MYSQL* mysql_init() {
  std::clog << __PRETTY_FUNCTION__ << std::endl;
  return new MYSQL;
}

void mysql_close(MYSQL* m) {
  std::clog << __PRETTY_FUNCTION__ << std::endl;
  delete m;
}

class Connection {
  boost::shared_ptr<MYSQL> m_mysql;
public:
  void connect() {
    m_mysql.reset(mysql_init(), mysql_close);
  }
  bool isConnected() const { return m_mysql.get() != 0; }
};

class ConnectionManager {
  std::map<std::string, Connection> m_connections;
public:
  static ConnectionManager* instance() {
    static ConnectionManager cm;
    return &cm;
  }
  Connection connection(const std::string& name = "default")
  { return m_connections[name]; }
};

int main()
{
  {
    Connection c = ConnectionManager::instance()->connection();
    std::clog << "isConnected " << c.isConnected() << std::endl;
    c.connect();
    std::clog << "isConnected " << c.isConnected() << std::endl;
  }
  Connection c = ConnectionManager::instance()->connection();
  std::clog << "isConnected " << c.isConnected() << std::endl;

  return 0;
}

And the output is:

isConnected 0
MYSQL* mysql_init()
isConnected 1
void mysql_close(MYSQL*)
isConnected 0

What I don't understand is why shared_ptr calls mysql_close too soon.
After all still exists a reference to Connection. I mean, mysql_close
should by called when the ConnectionManager singleton is destroyed or am
I doing something wrong.

- -- Filipe Sousa
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