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extern <data definition> ; [extern] <function prototype> ;
Use the extern modifier to indicate that the actual storage and initial value of a variable, or body of a function, is defined in a separate source code module. Functions declared with extern are visible throughout all source files in a program, unless you redefine the function as static.
The keyword extern is optional for a function prototype.
Use extern "C" to prevent function names from being mangled in C++ programs.
Also, extern templates allow you to define templates that are not instantiated in a translation unit. Using extern templates thus reduces both compilation time and the size of the compiled module. The extern template feature is part of the new C++0x standard.