c++ - Undefined reference error when using a simple class inside my function -

i getting nuts error thought of more experienced developers can me in regard. trying compile sample project uses c++ library (named poco). project linked compiled poco libraries.

below (most simplified) code:

#include "poco/uuid.h" class x {     void func1()                                             {                                                                                    new poco::uuid();  //a     }    };  void func1()                                         {                                                                                new poco::uuid();   //b } 

now when above code compiled, line 'a' has no error line 'b' linker says: undefined reference `poco::uuid::uuid()'

what reason? when instantiate class external lib in class method no error occurs same code in function produces linker error? (when comment line b, no error occurs , linker output files generated)

my configuration: win7/g++/codelite/mingw-4.7.1

*update 2:*thanks. problem resolved , issue had compiled library using msvc compiler while application being compiled using g++ (both under windows platform). re-compiled library using g++ , works fine now.

update 1: here ide's output when build project:

    c:\windows\system32\cmd.exe /c "mingw32-make.exe -j 4 -e -f  "dll1.mk"  all" ----------building project:[ dll1 - debug ]---------- g++ -shared -fpic -o ./debug/dll1.so @"dll1.txt" -l. -lc:/poco/lib  -lpocofoundationd ./debug/pluginlibrary.o: in function `z5func1v': c:/users/pars/documents/codelite/workspace1/dll1/pluginlibrary.cpp:12: undefined reference `poco::uuid::uuid()' collect2.exe: error: ld returned 1 exit status mingw32-make.exe: *** [debug/dll1.so] error 1 dll1.mk:77: recipe target `debug/dll1.so' failed 1 errors, 0 warnings 

your member function x::func1() never odr-used in compilation unit (source file). compilers generate compiled code member function defined inside class definition if member function odr-used within compilation unit being compiled. suppose other source file use x::func1(). if compile other source file, compiler produce object code x::func1() in object file corresponds other source file.

the compiler can away bypassing process of generating compiled code x::func1() here because class definition has same across compilation units. if compile other source file has different definition of class x have violated 1 definition rule. undefined behavior , no diagnosis required.

if no source file uses x::func1() have dead code never happens compiled. code has error it's never detected.

the compiler cannot away bypassing generating compiled code free function func1(). function has external linkage; there's no way compiler can tell if might used somewhere else. compiler must generate compiled code free function.

here's minimum working example:

class missing { public:    missing();    int value; };  class usesmissing { public:     int use_missing () {       missing missing;       return missing.value;    }     int dont_use_missing () {       return 0;    } };  #ifdef define_use_missing int use_missing () {    missing missing;    return missing.value; } #endif  int main () {    usesmissing test; #ifdef use_missing    return test.use_missing(); #else    return test.dont_use_missing(); #endif } 

compile neither define_use_missing or use_missing defined , compiles , links fine g++ , clang++. define either 1 of flags , file fails in link step because of undefined reference missing::missing().