Most common examples of misuse of singleton class

Question

When should you NOT use a singleton class although it might be very tempting to do so? It would be very nice if we had a list of most common instances of \'singletonitis\' t

Answer 1

Sometimes, you assume there will only be one of a thing, then you turn out to be wrong.

Example, a database class. You assume you will only ever connect to your app's database.

// Its our database! We'll never need another
class Database
{
};

But wait! Your boss says, hook up to some other guys database. Say you want to add phpbb to the website and would like to poke its database to integrate some of its functionality. Should we make a new singleton or another instance of database? Most people agree that a new instance of the same class is preferred, there is no code duplication.

You'd rather have

Database ourDb;
Database otherDb;

than copy-past Database and make:

// Copy-pasted from our home-grown database.
class OtherGuysDatabase
{
};

The slippery slope here is that you might stop thinking about making new instance of classes and instead begin thinking its ok to have one type per every instance.