I found that some c/c++ programs using extern keyword. What is this? And when/where will we use extern? Also, is extern "c" == extern? What different? Thx

you normally use extern in header files.

example: // this would be in game.h         extern int health; // can''t initialize only declare         // this allows you to use in any code or source file          // within that program think of it as a "super-global"         /*in game.c or game.cpp*/         int health = 100; // here is where you initialize it         // basically you can initialize it anywhere outside the         // header file it was declared in 

extern "C" allows you to use C-specific functions/code in C++ files ... sorry don''t have an example

--Alpha

lol... thanks!

--Alpha

quote:Original post by wah_on_2
I found that some c/c++ programs using extern keyword. What is this?

It''s a directive telling the compiler that the definition of the function or variable will be found outside of the current translation unit and, therefore, will be fixed-up by the linker.

quote:
And when/where will we use extern?

Anytime its undesirable to define the entity within the current translation unit, such as when you link to library functions.

quote:
Also, is extern "c" == extern? What different?

They''re similar, but extern "C" tells the compiler to apply the local C linkage convention to entities that the extern applies to. Without the "C", the compiler will default to C++ linkage conventions. This protocol enables linking of declarations compiled using a C++ compiler with binaries compiled with a C compiler. Furthermore, the C and C++ standards allow vendors to extend the convention to link to binaries written using other languages, and you might see things such as extern "FORTRAN" or extern "PASCAL" in some environments.