MESSAGE
| DATE | 2010-02-13 |
| FROM | Ruben Safir
|
| SUBJECT | Subject: [NYLXS - HANGOUT] C++ Workshop - side discussing on C++ variable declarations
|
Date: Sat, 13 Feb 2010 20:43:04 -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)
Alright - I'm top posting :)
Here is an example of the error that I had in mind, and you'll see how
the linker behaves, although I thought I might get a different behavior.
http://www.nylxs.com/docs/workshops/scope/
First
The header file construction: scope.h
---
#ifndef SCOPETEST
#define SCOPETEST
#endif
using namespace std;
extern int global_i;
int global_def;
void test_obj();
void test_obj2();
------
First Library File Construction: scope1.cc
-----
#include "scope.h"
#include
void test_obj(){
int global_i = 5;
global_def = 200;
cout << "Global Declared Variable ==>" << global_i << endl;
cout << "Global defined Variable ==>" << global_def << endl;
}
_______
Second Library Source File Construction: scope2.cc
______
#include "scope.h"
#include
void test_obj2(){
int global_i = 5;
global_def = 300;
cout << "Global Declared Variable ==>" << global_i << endl;
cout << "Global defined Variable ==>" << global_def << endl;
}
________
The main programming file: scope_main.cc
-----------
#include
#include "scope.h"
using namespace std;
int main(int argv, char * argc[]){
int global_i = 25;
// int global_def;
cout << "Global Declare " << global_i << endl;
cout << "Global Declare " << global_def << endl;
test_obj();
test_obj2();
return 0;
}
____________
Not create a makefile that produces two seperate object files, one for
each libary source file: makefile
_____________
scope : scope1.o scope2.o scope_main.o
g++ -o scope scope_main.o scope1.o scope2.o
scope1.o : scope1.cc scope.h
g++ -Wall -c scope1.cc
scope2.o : scope2.cc scope.h
g++ -Wall -c scope2.cc
file5_main.o : scope_main.cc
g++ -Wall -c scope_main.cc
_____________
Now compile
ruben-at-www2:/usr/local/apache/htdocs/nylxs/docs/workshops/scope> make
g++ -Wall -c scope1.cc
g++ -Wall -c scope2.cc
g++ -c -o scope_main.o scope_main.cc
g++ -o scope scope_main.o scope1.o scope2.o
scope1.o:(.bss+0x0): multiple definition of `global_def'
scope_main.o:(.bss+0x0): first defined here
scope2.o:(.bss+0x0): multiple definition of `global_def'
scope_main.o:(.bss+0x0): first defined here
collect2: ld returned 1 exit status
make: *** [scope] Error 1
Lets look at that output individually
The compiler creates all the objects (3 of them in this case) without a
problem:
g++ -Wall -c scope1.cc
g++ -Wall -c scope2.cc
g++ -c -o scope_main.o scope_main.cc
ruben-at-www2:/usr/local/apache/htdocs/nylxs/docs/workshops/scope> ls -al *o
-rw-r--r-- 1 ruben users 2272 2010-02-13 19:52 scope1.o
-rw-r--r-- 1 ruben users 2272 2010-02-13 19:52 scope2.o
-rw-r--r-- 1 ruben users 2312 2010-02-13 19:52 scope_main.o
ruben-at-www2:/usr/local/apache/htdocs/nylxs/docs/workshops/scope> nm scope1.o
000000c7 t _GLOBAL__I_global_def
00000087 t _Z41__static_initialization_and_destruction_0ii
00000000 T _Z8test_objv
U _ZNSolsEPFRSoS_E
U _ZNSolsEi
U _ZNSt8ios_base4InitC1Ev
U _ZNSt8ios_base4InitD1Ev
U _ZSt4cout
U _ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_
00000004 b _ZStL8__ioinit
U _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
U __cxa_atexit
U __dso_handle
U __gxx_personality_v0
00000000 B global_def
ruben-at-www2:/usr/local/apache/htdocs/nylxs/docs/workshops/scope> nm scope2.o
000000c7 t _GLOBAL__I_global_def
00000087 t _Z41__static_initialization_and_destruction_0ii
00000000 T _Z9test_obj2v
U _ZNSolsEPFRSoS_E
U _ZNSolsEi
U _ZNSt8ios_base4InitC1Ev
U _ZNSt8ios_base4InitD1Ev
U _ZSt4cout
U _ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_
00000004 b _ZStL8__ioinit
U _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
U __cxa_atexit
U __dso_handle
U __gxx_personality_v0
00000000 B global_def
ruben-at-www2:/usr/local/apache/htdocs/nylxs/docs/workshops/scope> nm scope_main.o
000000d3 t _GLOBAL__I_global_def
00000093 t _Z41__static_initialization_and_destruction_0ii
U _Z8test_objv
U _Z9test_obj2v
U _ZNSolsEPFRSoS_E
U _ZNSolsEi
U _ZNSt8ios_base4InitC1Ev
U _ZNSt8ios_base4InitD1Ev
U _ZSt4cout
U _ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_
00000004 b _ZStL8__ioinit
U _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
U __cxa_atexit
U __dso_handle
U __gxx_personality_v0
00000000 B global_def
00000000 T main
Here you can see that all three ojects made room for the symbol
global_def. I just don't know what the left colomn means, and the man
page for nm is not helpful.
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
>
> --
>
> Chris Knadle
> Chris.Knadle-at-coredump.us
--
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