[solved] Qt 4.7.0 MinGW undefined reference to extern symbols

[AlGaN]

Hi,

just installed Qt 4.7.0 MinGW (gcc version 4.4.0) and tried to compile a simple console example with QTextStream stdin/stdouts. I got undefined references when linking with MinGW/ld linker, as for Visual C++ 2008 all went fine.

#ifndef QSTD_H
#define QSTD_H
 
#include <QTextStream>
#include <QFile>
#include <QString>
 
/** @short helper objects and functions which help reduce the
       need for char[] and the standard library.  
 
    defines three @ref QTextStream instances
    which behave like the c++ standard iostreams, bound to the
    standard in/out/error. 
    
    Also provided, some helper functions for writing
    interactive stdin/stdout applications.
*/
//start
namespace qstd {
 
    /** @short  An alias for standard input
     */
    extern QTextStream cin;  /* declared only, defined in the .cpp file */
    /** @short  An alias for standard output
     */
    extern QTextStream cout;
    /** @short  An alias for standard error
     */
    extern QTextStream cerr;
    /** yes/no prompt
        interactive stdin UI - prompts user with
        a yes/no question. Repeatedly-asks
        until user supplies a valid answer.
 
        @param yesNoQuestion the yes/no question
        @return true/false depending on what the
        user responded.
    */
    bool yes(QString yesNoQuestion);
    /** Convenience function that feeds a  specific question
        to the yes() function.
        @usage do {.....} while(more ("foobar"));
        so that user sees the question: "Another foobar (y/n)? "
        @param name of the item being handled by the loop.
    */
    bool more(QString prompt);
    /** A function for safely taking an int from the keyboard.
        Takes data into a  QString and tests to make sure it
        can be converted to int before returning.
        @param base allows choice of number base.
        @return returns validated int.
    */
    int promptInt(int base = 10);
    /** A function for safely taking a double from the keyboard.
        Takes data into a  QString and tests to make sure it
        can be converted to double before returning.
        @return returns validated int.
    */
    double promptDouble();
    /** Complete dialog for opening a file for output.
        Asks user for file name, checks to see if
        file already exists and, if so, asks the user if
        it is ok to overwrite.
        @param Reference QFile parameter is set to point
        to the (eventually) opened file.
    */
    /** @short Dialog for a output file prompt
     */
    void promptOutputFile(QFile& outfile);
 
    /** @short Dialog for input file prompt */
    void promptInputFile(QFile& infile);
 
 
//end
}
 
#endif

#ifndef QSTD_H
#define QSTD_H

#include <QTextStream>
#include <QFile>
#include <QString>

/** @short helper objects and functions which help reduce the
       need for char[] and the standard library.  

    defines three @ref QTextStream instances
    which behave like the c++ standard iostreams, bound to the
    standard in/out/error. 
    
    Also provided, some helper functions for writing
    interactive stdin/stdout applications.
*/
//start
namespace qstd {

    /** @short  An alias for standard input
     */
    extern QTextStream cin;  /* declared only, defined in the .cpp file */
    /** @short  An alias for standard output
     */
    extern QTextStream cout;
    /** @short  An alias for standard error
     */
    extern QTextStream cerr;
    /** yes/no prompt
        interactive stdin UI - prompts user with
        a yes/no question. Repeatedly-asks
        until user supplies a valid answer.
 
        @param yesNoQuestion the yes/no question
        @return true/false depending on what the
        user responded.
    */
    bool yes(QString yesNoQuestion);
    /** Convenience function that feeds a  specific question
        to the yes() function.
        @usage do {.....} while(more ("foobar"));
        so that user sees the question: "Another foobar (y/n)? "
        @param name of the item being handled by the loop.
    */
    bool more(QString prompt);
    /** A function for safely taking an int from the keyboard.
        Takes data into a  QString and tests to make sure it
        can be converted to int before returning.
        @param base allows choice of number base.
        @return returns validated int.
    */
    int promptInt(int base = 10);
    /** A function for safely taking a double from the keyboard.
        Takes data into a  QString and tests to make sure it
        can be converted to double before returning.
        @return returns validated int.
    */
    double promptDouble();
    /** Complete dialog for opening a file for output.
        Asks user for file name, checks to see if
        file already exists and, if so, asks the user if
        it is ok to overwrite.
        @param Reference QFile parameter is set to point
        to the (eventually) opened file.
    */
    /** @short Dialog for a output file prompt
     */
    void promptOutputFile(QFile& outfile);

    /** @short Dialog for input file prompt */
    void promptInputFile(QFile& infile);
    
    
//end
}

#endif

To copy to clipboard, switch view to plain text mode 

//start id=namespace
#include "qstd.h"
 
/* QTextStreams look a lot like iostreams,
we just have to point them to the right place. */
 
//start id=streamdefs
QTextStream qstd::cin(stdin, QIODevice::ReadOnly);
QTextStream qstd::cout(stdout, QIODevice::WriteOnly);
QTextStream qstd::cerr(stderr, QIODevice::WriteOnly);
//end
 
[...]

//start id=namespace
#include "qstd.h"

/* QTextStreams look a lot like iostreams,
we just have to point them to the right place. */

//start id=streamdefs
QTextStream qstd::cin(stdin, QIODevice::ReadOnly);
QTextStream qstd::cout(stdout, QIODevice::WriteOnly);
QTextStream qstd::cerr(stderr, QIODevice::WriteOnly);
//end

[...]

To copy to clipboard, switch view to plain text mode 

#include <QCoreApplication>
#include <QtDebug>
#include "qstd.h"
 
 
using namespace qstd;
 
int main(int argc, char* argv[])
{
    QCoreApplication app(argc, argv);
 
    QString str1("Test");
 
    cout << str1 << endl;
 
    return 0;
}

#include <QCoreApplication>
#include <QtDebug>
#include "qstd.h"


using namespace qstd;

int main(int argc, char* argv[])
{
    QCoreApplication app(argc, argv);
    
    QString str1("Test");

    cout << str1 << endl;
    
    return 0;
}

To copy to clipboard, switch view to plain text mode 

When compiling with Qt 4.7.0/MinGW I get linker error:

mingw32-make[1]: Entering directory `D:/projekt/work/test/qstring_test'
g++ -enable-stdcall-fixup -Wl,-enable-auto-import -Wl,-enable-runtime-pseudo-re
oc -Wl,-subsystem,console -mthreads -Wl -o debug\qstring_test.exe debug/qstd.o
ebug/qstring_test.o  -L"d:\Qt\2010.05\qt\lib" -lQtCored4
debug/qstring_test.o: In function `main':
D:\projekt\work\test\qstring_test/qstring_test.cpp:14: undefined reference to `
qstd::cout'
collect2: ld returned 1 exit status
mingw32-make[1]: *** [debug\qstring_test.exe] Error 1
mingw32-make[1]: Leaving directory `D:/projekt/work/test/qstring_test'
mingw32-make: *** [debug] Error 2

mingw32-make[1]: Entering directory `D:/projekt/work/test/qstring_test'
g++ -enable-stdcall-fixup -Wl,-enable-auto-import -Wl,-enable-runtime-pseudo-re
oc -Wl,-subsystem,console -mthreads -Wl -o debug\qstring_test.exe debug/qstd.o
ebug/qstring_test.o  -L"d:\Qt\2010.05\qt\lib" -lQtCored4
debug/qstring_test.o: In function `main':
D:\projekt\work\test\qstring_test/qstring_test.cpp:14: undefined reference to `
qstd::cout'
collect2: ld returned 1 exit status
mingw32-make[1]: *** [debug\qstring_test.exe] Error 1
mingw32-make[1]: Leaving directory `D:/projekt/work/test/qstring_test'
mingw32-make: *** [debug] Error 2

To copy to clipboard, switch view to plain text mode 

What am I doing wrong? Thanks for any hints

[nish]

what happens when u use qstd::cout ...? the errors shows that "cout" is treated as std::cout by mingw.

[AlGaN]

Hi,

sorry, that was a cut&paste error, the error states qstd:cout of course...
Compiling with Visual C++ 2008 does not show any error, so it must be something with MinGW/gcc I suppose

[nish]

all seems to be correct... try to put all of the code in qstd.cpp file inside the qstd namespace and remove qstd:: from the defination.

[AlGaN]

Hi MrDeath

thanks! That works! Now I wonder why there is a difference in MinGW in defining symbols in a namespace and full qualifiying them in the signature?
Anyway, thanks for your help!