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 11:35:23 -0500
From: Chris Knadle
To: NYLUG Technical Discussion
Subject: Re: [nylug-talk] C++ Workshop - side discussing on C++ variable
declarations
User-Agent: KMail/1.12.4 (Linux/2.6.32.7-c2d-crk6; KDE/4.3.4; x86_64; ; )
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
--
Chris Knadle
Chris.Knadle-at-coredump.us
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