C++ Yet another linker problem.

Question

Hi all.

I'm having a linking issue with a basic C++ program. No, I'm not including .cpp files!

This is what's happening.

main.cpp

#include "header.h"
#include <iostream>

int main() {
   std::cout << "Hello!";
}

header.h

#ifndef _HEADER_H
#define _HEADER_H

class Something {
public:
   printContents();
};

#endif

something.cpp

#include "header.h"

#include <iostream>

Something::printContents() {
    cout << "This class's Contents!!";
}

What's happening is that I get a compiler error going: multiple definitions of some standard C function, such as strtod.

g++ -o ... main.o
build/....main.o: In function `strtod':

../MinGW/bin/../lib/gcc/mingw32/3.4.5/../../../../include/stdlib.h:318: multiple definition of `strtod'

build/..something.o:...something.cpp:(.text+0x0): first defined here collect2: ld returned 1 exit status

If I get rid of #include in one of the two occasions and get rid of the couts, it will compile. What the hell? I'm using g++ and NetBeans to compile.

I tried in the command line: g++ *.h *.cpp -o program

and the same thing happened.

Thanks!

Answer 1

One of your problems is right here:

I tried in the command line: g++ *.h *.cpp -o program

Don't pass your header files... Try something like this:

g++ *.cpp -o program

Answer 2

Please note that _HEADER_H is an illegal name in C++ user code - names beginning with the underscore and an uppercase letter are reserved for the C++ implementation. This does not normally cause noticeable problems, but when you use what may be a common name in the implementation like HEADER in this context, it well might.

Answer 3

Modify,

Something::printContents() 
{    
  std::cout << "This class's Contents!!";
}

NOTE: Specify the return datatype.

Answer 4

I could not reproduce your exact problem. I get this to compile and link nicely with the following few notes:

• Add a void return type to the printContents-function (So it says void printContents(); in the header and void Something::printContents() { in the implementation-file)
• Use std::cout rather than just cout. cout is not defined in the scope it is used
• Make sure header.h ends with a blank line
• Use HEADER_H rather than _HEADER_H (like Neil Butterworth says)

I use the command line g++ main.cpp something.cpp to compile.

Answer 5

I see a couple of problems. You shuold define the returning value of the function

printContents()

and you must write

std::cout

if you don't write

using namespace std;

Answer 6

The problem was in a multi-installation of MinGW. I had one already installed, and when I got Qt on my computer, it had installed it's own MinGW. Bummer, I ported the code to my university's servers and it ran OK.

Bummer!!

Thanks everybody for the help, I will definitely follow your guidelines in the future.

Header names - no underscores Correct return type Real code in the forums!

Leo Bruzzaniti