Hi to all, long time no see!!!

I have a question now:

In my project I will need variable that will hold the database connection. The variable will be initialized at application startup and all for objects in the application it is a must to see it. How and where do I put such a variable, that must be seen by all objects in an application? I want to follow c++ rules and I heard global variables are BAD idea. I also want to know why are global variables such a bad idea?