667
edits
Changes
m
→nsISupports
[http://developer.mozilla.org/en/docs/nsISupports nsISupports] is the base interface for all XPCOM components (i.e., it is possible to pass any XPCOM component around as an nsISupports object). The [http://developer.mozilla.org/en/docs/nsISupports methods] in nsISupports define basic bookkeeping for an interface's lifetime (they also defines a way to check at runtime if a component implements a given interface, but more on that later).
Components need to keep track of how many clients hold references to them via an interface. This is known as '''reference counting ''' on an interface, and it is used to determine when a component can be safely unloaded so that it doesn't leak (i.e., no one holds a reference any more, but the interface is still in memory).
The members of nsISupports (i.e., QueryInterface, AddRef, and Release) provide the basic means for getting the right interface from an object, incrementing the reference count, and releasing objects once they are not being used.
One point worth mentioning is that '''pointers in XPCOM are to interfaces'''. Interface pointers are known to implement nsISupports, so you can access all of the object lifetime and discovery functionality described above.
So the first line above says, "include the interface for nsISupports (defined in nsISupports.idl) because I'll need it", and the fourth line says, "I'm a new interface called IFirstXpcom, but I'm also nsISupports because I inherit from it."