This reference describes the interfaces and functions provided by the XPCOM library. In addition, it details the various helper classes and functions, as well as the components, provided by the XPCOM glue library. The contents herein are oriented primarily toward extension developers and people embedding XPCOM in other projects.
Note: If you're working on a module in the Mozilla codebase that's compiled with the MOZILLA_INTERNAL_API flag set, some of these APIs -- the string functions and classes in particular -- are not the ones you should be using. See the XPCOM internal string guide for documentation of the internal string API used within the Mozilla codebase.
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Many XPCOM pages return an nsresult. Prior to Gecko 19 (Firefox 19 / Thunderbird 19 / SeaMonkey 2.16), this was an integer that simply returned an error code. It is now a strongly typed enum when XPCOM is built using a C++11 compiler. This causes compile-time errors to occur when improper values are returned as nsresult values, thereby making it easier to catch many bugs.