undefined reference to...

undefined reference to...

[Philippe De Neve]

Hi,

I'm new to programming and I'm having a problem I don't understand:

I have 3 very simple files:

1) Integerclass.h containing :

class Integer{
        int i;
public:
        Integer(int j){
                i=j;
        }
        void change(void);
};

2) Integerclass.cpp containing :


#include "Integerclass.h"

void Integer::change(void){
        i=5;
}

3) my_prog.cpp containing

#include "Integerclass.h"

int main(){
        Integer P(2);
        P.change();
        return 0;
};

When I compile the Integerclass.cpp file no errors are returned.

But when I compile and link this :

c++ -o my_prog my_prog.cpp

the output is :

demovideo3:/Projects/little_proggie# c++ -o my_prog my_prog.cpp
/tmp/ccsGvVWq.o: In function `main':
/tmp/ccsGvVWq.o(.text+0x1f): undefined reference to `Integer::change(void)'
collect2: ld returned 1 exit status
demovideo3:/Projects/little_proggie#

I do not understand where I'm making a mistake. I did the same on a windows
machine using VC++ and no errors where returned? Can anyone explain me what
I'm doing wrong? Any help is appreciated!

regards, Philippe.










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Re: undefined reference to...

[Steven Smith]

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> 1) Integerclass.h containing :
...
> 2) Integerclass.cpp containing :
...
> 3) my_prog.cpp containing
...
> But when I compile and link this :
> 
> c++ -o my_prog my_prog.cpp
That command says to compile my_prog.cpp to an object and use only
that object, plus the standard libraries, to build my_prog i.e.
ignore Integerclass.cpp completely.  There are two obvious solutions:

1) Compile everything at once:

c++ -o my_prog my_prog.cpp Integerclass.cpp

2) Compile the source files seperately, and then link them together

c++ -c my_prog.cpp
c++ -c Integerclass.cpp
c++ -o my_prog my_prog.o Integerclass.o

Hope that helps.

Steven Smith,
sos22@cantab.net.
-- 
One day, I'm going to get an Alice-bot to answer all my email for me,
and see how long it takes people to notice.

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Re: undefined reference to...

[Ray Olszewski]

At 04:47 PM 8/20/2004 +0200, Philippe De Neve wrote:
>Hi,
>
>I'm new to programming and I'm having a problem I don't understand:
>
>I have 3 very simple files:
>
>1) Integerclass.h containing :
>
>class Integer{
>         int i;
>public:
>         Integer(int j){
>                 i=j;
>         }
>         void change(void);
>};
>
>2) Integerclass.cpp containing :
>
>
>#include "Integerclass.h"
>
>void Integer::change(void){
>         i=5;
>}
>
>3) my_prog.cpp containing
>
>#include "Integerclass.h"
>
>int main(){
>         Integer P(2);
>         P.change();
>         return 0;
>};
>
>When I compile the Integerclass.cpp file no errors are returned.
>
>But when I compile and link this :
>
>c++ -o my_prog my_prog.cpp
>
>the output is :
>
>demovideo3:/Projects/little_proggie# c++ -o my_prog my_prog.cpp
>/tmp/ccsGvVWq.o: In function `main':
>/tmp/ccsGvVWq.o(.text+0x1f): undefined reference to `Integer::change(void)'
>collect2: ld returned 1 exit status
>demovideo3:/Projects/little_proggie#
>
>I do not understand where I'm making a mistake. I did the same on a windows
>machine using VC++ and no errors where returned? Can anyone explain me what
>I'm doing wrong? Any help is appreciated!
>
>regards, Philippe.

Since VC++ is GUI based, I doubt you really did the "same" using it. I 
mention this only because as a beginner, you need too learn the importance 
of giving exact reports, not impressionistic ones, when asking for help or 
advice.

Your problem is that the way you compile using c++, you do not give the 
linker (ld, invoked by c++ to link after the compile is done) any 
information about where to find the intermediate code for Integerclass.cpp. 
If you run c++ this way instead --

          c++ -o my_prog my_prog.cpp Integerclass.cpp

-- you should get a successful compile/link. (At least I do here, though 
when run the program doesn't actually do anything visible, which looks to 
be consistent with what you've actually coded.) But this is just a 
quick-and-dirty solution to your immediate problem, not the real answer you 
need.

More generally, what you are missing is the use of "make" to manage 
multi-file compiles with gcc, c++, and the rest of the Gnu compiler set. I 
haven't used VC++ in years, but I expect it has an equivalent to make 
hidden somwehere in the programming environment it offers (I know the 
Borland C++ compiler suite does; I've used that more than VC++).

An e-mail is no place for a tutorial on make, but there are plenty of 
resources that introduce it and its use. Look around in the usual places.

BTW, how did you compile Integerclass.cpp by itself? Since it is not a 
standalone program, you'd need to use some of the c++ switches to make the 
compile work (the natural one is "c++ -c Integerclass.cpp"), and that 
suggests you know more about using c++ than I'd been thinking here.



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