Re: question about extern ?..

From: JKop (NULL_at_NULL.NULL)
Date: 06/08/04 

Date: Tue, 08 Jun 2004 15:44:40 GMT

mark posted:

> compiler g++ on linux
>
> file.h has the following, int x=5;
> and inside the main program file I can access "x" without declaring it
> "extern x". Program compiles and runs fine....
>
> also program compiles and runs fine with x declared as extern.
>
> So is it necessary to declare x as extern if one get away without
> declaring it extern ? What is the standard and good practice ?

First of all, let's say you have a CPP file as so:

#include "blah.hpp"

int main(void)
{
            some_variable = 7;
}

After the preprocessor has processed that, you'll have a translation unit:

extern int some_variable;

int main(void)
{
            some_variable = 7;
}

Now, the thing here is that no variable is declared at all!! The "extern"
says "Don't actually make a variable. There's already one made somewhere
else in some other CPP file. Compile *ME*, _this_ CPP file, and then let the
linker bother with finding out where the variable was actually declared!"

So our CPP file gets compiled. Now, somewhere else, you _MUST_ have CPP file
containing:

int some_variable;

So now, the linker takes all your translation units, which the compiler has
turned into object files, and sorts it all out.

If you DON'T want a particular variable to be visible outside of your CPP
file, then:

static int some_variable;

In this case, you won't get a compile error at all, but a Link errror, the
linker can't find the variable to which your "extern" statement refers.

-JKop

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