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= Writing FirstXpcom =
For this walkthrough we will use the Mozilla build system to create our component. It is also possible to us use the '''Gecko SDK''' ([http://developer.mozilla.org/en/docs/How_to_build_a_binary_XPCOM_component_using_Visual_Studio instructions are here]). NOTE: this assumes that you have already done a successful objdir-Firefox build.
== Creating Directories ==
=== 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 define 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).
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Now [http://developer.mozilla.org/En/Generating_GUIDs write a define ] to make it easier to pass this Class ID aroundin C++:
#define FIRSTXPCOM_CID \
