Researching XPIDL and IDL Technologies

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XPIDL stands for XP Interface Description Language. It is a specification for XPCOM which is a cross platform adapter description language. An adapter description language is used to describe an adapter which unrelated with its machine language. The description of the adapter can use specialized tools to handle automatic generation of language that is unrelated to the adapter's specifications. Typically XPIDL is frequently used to generate C++ header files and typelib information.



An interface is declared with the interface keyword, and the simplest possible interface is a declaration without any methods or attributes.

 interface nsToolbar {

To specify an interface's parent, follow the interface name with a colon and the parent name

interface nsIToolbar : nsIParent {


An interface can have both methods and attributes. Attributes are properties of interface objects that can be read and optionally set. The following example - Attributes, properties of interface objects, can be read and optionally set. - This is an example of an interface with one method (no return value, no parameters) named fun and an integer-valued attribute named attr

interface nsIFoo {
  attribute long attr;
  void fun();

Methods can have any number of in, out or inout parameters, with a variety of types.

In this example many parameters of different types and in an out specifications are seen

interface nsIStringStuff {
  void findStringLength(in string str, out long l);
  void concatenateStrings(in string str1, in string str2,
                         out string result);
  void replaceChar(inout string str, in char from, in char to,
                  in boolean foldCase);

Attributes can be made read-only, by placing the read-only keyword infront of the the definition

interface nsIThing {
  readonly attribute string lookButDontTouch;
C++ get and set methods for attributes are automatically declared for attributes in the   


The compiler capitalizes method names when generating C++ headers. It supports existing C++ convention in Mozilla, used the InterCaps method naming style. The method name used in XPIDL will appear with the same capitalization to JavaScript, so the best practice is to match JavaScript convention by declaring attributes and method names in InterCaps.

Built In Types

Type                    C++ 
void 	                void
boolean 	        PRBool
octet 	                PRUint8
short 	                PRInt16
long 	                PRInt32
long long 	        PRInt64
unsigned short 	        PRUint16
unsigned long 	        PRUint32
unsigned long long 	PRUint64
float 	                float
double 	                double
char           	        char
wchar 	                PRUnichar
string 	                char*
wstring 	        PRUnichar*

Native Types

     It is possible to use native types in XPIDL by declaring then using the native declaration syntax which works similar to a typedef in C++.
A native type can be given a name using the following syntax:
     native name(native_type);

Reference and pointer types can be defined by combining [ref] or [ptr] with native languages

XPIDL Code                                                Generated C++
[ref] native nsNativeFileRef(nsFileSpec);                 /* starting interface:  foo */
[ptr] native nsNativeFilePtr(nsFileSpec);                 class foo {
                                                           /* void openByRef (in nsNativeFileRef aFileSpecRef); */
                                                           NS_IMETHOD OpenByRef(nsFileSpec & aFileSpecRef) = 0;
                                                           /* void openByPtr (in nsNativeFilePtr aFileSpecPtr); */ 
                                                           NS_IMETHOD OpenByPtr(nsFileSpec * aFileSpecPtr) = 0;
interface foo {
  void openByRef(in nsNativeFileRef aFileSpecRef);
  void openByPtr(in nsNativeFilePtr aFileSpecPtr);

Native types in your code can be a good way to migrate existing interfaces. However, using native types in a method will mean that method is not scriptable.

Include Terminology

To referencing interfaces defined in other files, you must type #include "filename.idl".
In XPIDL, #include only includes the file once, so the #include statement does not need the ifdefs which will be required for the    
the C preprocessor.

Enumerations and Constants

C-style enumerations are not supported in XPIDL because the size of enums can be compiler specific. XPIDL alternatively supports defining constants that appear in the C++ and JavaScript mappings. They must be of type short or long and have to be present inside an interface declaration. Other types are not supported because they do not map into all languages or have memory management issues. To define a constant that is neither short or long, write a method that returns the value, or include the value in an attribute.

XPIDL Code                                      C++ Code
const short c1 = 1+1;                           enum { c1 = 2 };
const short c2 = c1 * 5;                        enum { c2 = 10 };
const long flag = 1 << 5;                       enum { flag = 32 };
const float invalid_constant = 6.0;             Warning: const decl 'invalid_constant' was not of type short or long, ignored




These groups typically have topics that relate to XPIDL.


Frequently Asked Questions about XPConnect and XPIDL

Documentation at Mozilla Development Centre

Rules And Syntax