Access Mysql * connection variable from different c++ class - c++

Question

I wrote c++ class to connect to mysql database:
hpp file
#include <vector>
#include <string>
#include "mysql/mysql.h"
#ifndef _DATA
#define _DATA
class Database {
public:
string host;
string user;
string pwd;
string db;
MYSQL * connection;
MYSQL_RES *result;
MYSQL_ROW row;
Database(const string & host,
const string & user,
const string & pwd,
const string & db);
int createMysqlConnection();
};
#endif
cpp file
#include "Database.hpp"
Database::Database(const string & host,
const string & user,
const string & pwd,
const string & db) :
mysqlHost(host),
mysqlUser(user),
mysqlPassword(pwd),
mysqlDBName(db)
{}
int Database::createMysqlConnection(){
MYSQL * connection;
connection = mysql_init(NULL);
if(!mysql_real_connect(connection, mysqlHost.c_str(), mysqlUser.c_str(),
mysqlPassword.c_str(), mysqlDBName.c_str(),
0, NULL, 0)){
fprintf(stderr, "Connection to database failed: %s\n",
mysql_error(connection));
return EXIT_FAILURE;
}
cout << "connected to mysql" << endl;
};
When I'm trying to access connection variable from the main function or from a different class I always get an error like variable 'connection is not declared in this scope. I tried to use friend classes or inheritance to point to connection variable but it didn't work. I think I'm doing something wrong in my syntax.
Here is an example of how I try to point to this variable from different class:
Class risk: public Database {
public:
vector<sting> parameter;
Datasabse.connection;
etc....
}

Answer 1

to solve this problem i had to define MYSQL connection in class constructor. here is a code sample:
class1.hpp
***headers***
class Flight {
public:
Flight(MYSQL * connection);
MYSQL * c;
MYSQL_RES * res;
etc...
Class1.cpp
***headers***
Flight::Flight (MYSQL* connection)
{
c = connection;
};
etc...
main.cpp
***headers***
int main (int argc, char *argv[]) {
Database db(localhost, user, etc..);
db.createMysqlConnection;
Flight fl(db.connection);
fl.GetIDs();
etc...

Answer 2

MYSQL * connection;
declares the connection variable as local to the constructor only. You must move it to the class definition in order to make it available somehow. Alternatively, make a connect method in your Database object to explicitely create a new connection adlib. You will have to mirror it with a close method.
Yet another option is to create a dedicated class Connection for connections, which would:
be constructed with a MYSQL *,
be created by the connect method of Database as suggested above; the Database class would act as a Connection factory,
sports all the necessary methods to handle it,
and would call mysql_close in its destructor, to ensure proper termination of the underlying connection.
For instance:
class Connection {
protected:
MYSQL *connection;
public:
Connection(MYSQL *c): connection(c){}
virtual ~Connection() { mysql_close(connection); }
MYSQL_STMT * prepareQuery(const string &query) {
return mysql_prepare(connection, query.c_str(), query.length());
}
// other methods
};
The problem with this approach is that you quickly feel compeled to wrap all the API primitives with classes.