MESSAGE
| DATE | 2010-02-13 |
| FROM | Ruben Safir
|
| SUBJECT | Subject: [NYLXS - HANGOUT] C++ Workshop - side discussing on C++ variable declarations
|
Date: Fri, 12 Feb 2010 17:47:04 -0500
From: Ruben Safir
To: NYLUG Technical Discussion
Subject: Re: [nylug-talk] C++ Workshop - side discussing on C++ variable
declarations
User-Agent: Mutt/1.5.20 (2009-06-14)
On Fri, Feb 12, 2010 at 11:35:23AM -0500, Chris Knadle wrote:
> On Friday 12 February 2010 03:00:23 Chris Knadle wrote:
> > On Thursday 11 February 2010 19:24:44 Ruben Safir wrote:
> > > On Thu, Feb 11, 2010 at 07:01:11PM -0500, Ajai Khattri wrote:
> > > > On Thu, 11 Feb 2010, Ruben Safir wrote:
> > > > > Anyone have any constructive insight on this conversation I had with
> > > > > a friend?
> > > >
> > > > I dont see what the problem is. If you're linking to code that is
> > > > outside the current file you must tell the compiler that it is defined
> > > > EXTERNally.
> > > >
> > > > Often when your code is spread across several files, you might want to
> > > > refer to symbols defined in other files or other libraries, so you must
> > > > declare them ahead of your code.
> > >
> > > Its the idea that if you define the var (that is, not use the extern
> > > keyword), that it can create a problem that is bothering me and getting
> > > under my skin.
>
> I've created a C++ program to illustrate, as well as to test the way I thought
> this worked.
>
> //==================================
> //file extern_test_1.cpp
>
> #include
>
> using namespace std;
>
> int global_var=0;
> void external_increment(void); // extern funct to increment global_var by 1
>
> void local_increment_10(void)
> {
> global_var += 10;
> cout << global_var << "\t after +10 locally" << endl;
> }
>
> int main(void)
> {
> for (int x=0; x<4; x++) {
> local_increment_10(); // global_var += 10
> external_increment(); // global_var += 1
> external_increment(); // global_var += 1
> }
> cout << "ending global_var: " << global_var << endl;
> return 0;
> }
> //==================================
>
>
>
> //==================================
> // file extern_test_2.cpp
>
> #include
>
> using namespace std;
>
> extern int global_var;
>
> void external_increment(void)
> {
> global_var += 1;
> cout << global_var << " \t after +1 externally" << endl;
> }
> //==================================
>
>
>
> Compiled via the command:
> g++ extern_test_1.cpp extern_test_2.cpp -o extern_test
>
>
>
> Notes:
> - Leaving out the 'extern' keyword in extern_test_2.cpp causes
> a compile error. (This is a good thing!)
>
> - In the function prototype for external_increment() in
> extern_test_1.cpp I didn't add the 'extern' keyword because I
> wanted to demonstrate that for functions (only) the 'extern'
> keyword is optional in C++, but it should /should/ be added
> there for human readability. The compiler may be able to
> figure out the function is external, but I wouldn't want to
> have to...
>
> - Global variables should be avoided where possible
>
> -- Chris
>
I was just about to do something like this, but Shabbos is coming. Your
demonstration shows how extern works, which is good. What I wan't to do
is create a program with a header file that passes compilation without
the extern, but fails programatically in order to demonstrate how by not
using extern, that C++ becomes completely confused. It will require two
source files that use the same header and then a finished binary that
uses functions from each source file what have unpredictable results.
I also wrote a ton of stuff that I haven't posted yet, relating to
syntax and data types, and scope.
I'll get to it Saturday night. Signing off for now.
Ruben
> --
>
> Chris Knadle
> Chris.Knadle-at-coredump.us
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