Unresolved external symbol in object files

Question

During coding in Visual Studio I got an unresolved external symbol error and I ve got no idea what to do  I don t know what s wrong  Could you please decipher me  Where should I be looking for what kind of errors   1 gt Form obj   error LNK2019  unresolved external symbol  public  class Field     thiscall Field  addField class Field       addField Field  QAEPAV1 PAV1  Z  referenced in function  public  void   thiscall Form  parse class std  basic stringstream lt char struct std  char traits lt char gt  class std  allocator lt char gt   gt   amp      parse Form  QAEXAAV  basic stringstream DU  char traits D std  V  allocator D 2  std   Z  1 gt Form obj   error LNK2019  unresolved external symbol  public  virtual void   thiscall Field  parse class std  basic stringstream lt char struct std  char traits lt char gt  class std  allocator lt char gt   gt   amp      parse Field  UAEXAAV  basic stringstream DU  char traits D std  V  allocator D 2  std   Z  referenced in function  public    thiscall InputField  InputField class std  basic stringstream lt char struct std  char traits lt char gt  class std  allocator lt char gt   gt   amp       0InputField  QAE AAV  basic stringstream DU  char traits D std  V  allocator D 2  std   Z  1 gt Form obj   error LNK2001  unresolved external symbol  public  virtual void   thiscall Field  prompt void     prompt Field  UAEXXZ  1 gt Form obj   error LNK2001  unresolved external symbol  public  virtual class std  basic string lt char struct std  char traits lt char gt  class std  allocator lt char gt   gt    thiscall Field  getName void     getName Field  UAE AV  basic string DU  char traits D std  V  allocator D 2  std  XZ  1 gt Form obj   error LNK2001  unresolved external symbol  public  virtual class std  basic string lt char struct std  char traits lt char gt  class std  allocator lt char gt   gt    thiscall Field  getType void     getType Field  UAE AV  basic string DU  char traits D std  V  allocator D 2  std  XZ  1 gt Form obj   error LNK2001  unresolved external symbol  public  virtual void   thiscall Field  describe void     describe Field  UAEXXZ  1 gt C  Users tomy Documents Visual Studio 2010 Projects zapoctovkac   Debug zapoctovkac   exe   fatal error LNK1120  6 unresolved externals

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See Linker Tools Error LNK2019 at MSDN  it has a detailed list of common problems that cause LNK2019

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In my case  I needed add the function name to the DEF file  LIBRARY   DEMO EXPORTS    ExistingFunction    1    MyNewFunction       2

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I m doing some C   for the first time in a long time  and I m getting this error when I forget to add the ClassName   prefix for the function definition  since this is a little unique to C    So remember to check for that too

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I had an error where my project was compiled as x64 project  and I ve used a Library that was compiled as x86   I ve recompiled the library as x64 and it solved it

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I believe most of the points regarding the causes and remedies have been covered by all contributors in this thread   I just want to point out for my  unresolved external  problem  it was caused by a datatype defined as macro that gets substituted differently than expected  which results in that incorrect type being supplied to the function in question  and since the function with type is never defined  it couldn t have been resolved   In particular  under C C   -  Language  there is an attribute called  Treat WChar t As Built in Type  which should have been defined as  No   Zc wchar t-   but did not in my case

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I ve just had the same error and I manage to avoid it by replacing   with    in the header file    ifndef XYZ h  define XYZ h class XYZ       public      void xyzMethod        endif   When it was void xyzMethod    it didn t want to compile

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sometimes if a new header file is added  and this error starts coming due to that  you need to add library as well to get rid of unresolved external symbol   for example     include WtsApi32 h   will need     pragma comment lib   Wtsapi32 lib

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It looks to be missing a library or include   you can try to figure out what class of your library that have getName  getType etc     and put that in the header file or using  include   Also if these happen to be from an external library  make sure you reference to them on your project file   For example  if this class belongs to an abc lib then in your Visual Studio    Click on Project Properties   Go to Configuration Properties  C C    Generate  verify you point to the abc lib location under Additional Include Directories  Under Linker  Input  make sure you have the abc lib under Additional Dependencies

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Just spent a couple of hours to find that the issue was my main file had extension  c instead of  cpp

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I came here looking for a possible explanation before taking a closer look at the lines preceding the linker error   It turned out to have been an additional executable for which the global declaration was missing

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Check you are including all the source files within your solution that you are referencing    If you are not including the source file  and thus the implementation  for the class Field in your project it won t be built and you will be unable to link during compilation   Alternatively  perhaps you are using a static or dynamic library and have forgotten to tell the linker about the  libs

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I just had a hard time with this  Everything was logically set up  I declared a constructor but didn t define it  class SomeClass      SomeClass        needs the SomeClass  SomeClass     function defined somewhere  even here     I almost banged my head on my keyboard when I forgot something so elementary

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Yet another possibility to check  it was my problem this time   I had added the function to the library  and included the library s output folder in the search path    But I also had a folder with an older version of the library listed before  so VS was using the old library  and of course not finding the new function

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Make sure you decorate your header files with    ifndef YOUR HEADER H  define YOUR HEADER H     your header code here   endif   Bad things -including this- can happen if you don t

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I faced a similar issue and finally managed to solve it by adding   declspec dllimport  to the declaration of the class     A hpp class   declspec dllimport  A      public  void myFunc           Function declaration

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What had caused it in my case   I had a huge file Foo cpp without a Foo h  Foo cpp began like this          100 LOC here     namespace NS          100 more LOC here     static int var    I removed the  static  keyword and added a Foo h with this   extern int var    Do you see the mistake   I totally missed that var was originally defined in a namespace  because the namespace declaration was buried in other code  The fix is to change the extern like this   namespace NS        extern int var

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My issue was  I had to do forward declaration of the class whose ctor was  unresolved external    In the file where I got the error  I had to put something like this    include  ClassB    class ClassB     this solved the problem  class ClassA      void foo            ClassB  tmp   new ClassB                              Of course  my project is much more complicated and this is just a snippet example  Also when using namespaces  declare them as well

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I ve just seen the problem I can t call a function from main in  cpp file  correctly declared in  h file and defined in  c file  Encountered a linker error  Meanwhile I can call function from usual  c file  Possibly it depends on call convention  Solution was to add following preproc lines in every  h file    ifdef   cplusplus extern  C     endif   and these in the end   ifdef   cplusplus    endif

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POINTERS  I had this problem and solved it by using pointer  I see that this wasn t your issue but I thought I d mention it because I sure wish it had been here when I saw this an hour ago  My issue was about declaring a static member variable without defining it  the definition needed to come after some other set ups  and of course a pointer doesn t need a definition  Equally elementary mistake  P

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My issue was a sconscript did not have the cpp file defined in it  This can be very confusing because Visual Studio has the cpp file in the project but something else entirely is building

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In addition to the excellent answer by Chris Morris above  I found a very interesting way you can receive this same fault if you are calling to a virtual method that hasn t been set to pure but doesn t its own implementation   It is the exact same reason  the compiler can t find an implementation of the method and therefore crooks   but my IDE did not catch this fault in the least bit   for example  the following code would get a compilation error with the same error message      code testing an interface class test      void myFunc          define an interface class IamInterface       virtual void myFunc         implementation of the interface class IamConcreteImpl       void myFunc                1 1 2            However  changing IamInterface myFunc   to be a pure virtual method  a method that  must  be implemented  that than a virtual method which is a method the  can  be overridden  will eliminate the compilation error     define an interface class IamInterface       virtual void myFunc     0      Hopes this helps the next StackOverFlow person stepping through code

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This error often means that some function has a declaration  but not a definition   Example      A hpp class A   public    void myFunc       Function declaration        A cpp     Function definition void A  myFunc          do stuff     In your case  the definition cannot be found  The issue could be that you are including a header file  which brings in some function declarations  but you either    do not define the functions in your cpp file  if you wrote this code yourself  do not include the lib dll file that contains the definitions    A common mistake is that you define a function as a standalone and forget the class selector  e g  A    in your  cpp file   Wrong  void myFunc        do stuff      Right  void A  myFunc        do stuff

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This error can be caused by putting the function definitions for a template class in a separate  cpp file  For a template class  the member functions have to be declared in the header file  You can resolve the issue by defining the member functions inline or right after the class definition in the  h file  For example  instead of putting the function definitions in a  cpp file like for other classes  you could define them inline like this  template lt typename T gt  MyClassName     void someFunction            Do something               void anotherFunction            Do something else                Or you could define them after the class definition but in the same file  like this  template lt typename T gt  MyClassName     void someFunction      void anotherFunction       void MyClassName  someFunction          Do something         void MyClassName  anotherFunction          Do something else          I just thought I d share that since no one else seems to have mentioned template classes  This was the cause of the error in my case

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Yet another possible problem  that I just scratched my head about for some time    If you define your functions as inline  they   of course    have to be defined in the header  or an inline file   not a cpp  In my case  they were in an inline file  but only because they were a platform specific implementation  and a cpp included this corresponding inl file    instead of a header  Yeah  s  t happens   I thought I d leave this here  too  maybe someone else runs into the same issue and finds it here

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Make sure that you are not trying to overload the insertion or extraction operators as inline functions  I had this problem and it only went away when i removed that keyword

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One possible cause of this linker error can also be inline functions that are declared but not defined in a header file that is then included somewhere else  Inline functions have to be defined in every translation unit they are used in

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I had the same link errors  but from a test project which was referencing another dll  Found out that after adding   declspec dllexport  in front of each function which was specified in the error message  the link was working well

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A possible reason for the  Unresolved external symbol  error can be the function calling convention   Make sure that all the source files are using same standard   c or  cpp   or specify the calling convention   Otherwise  if one file is a C file  source c  and another file is a  cpp file  and they link to the same header  then the  unresolved external symbol  error will be thrown  because the function is first defined as a C cdecl function  but then C   file using the same header will look for a C   function    To avoid the  Unresolved external symbol error   make sure that the function calling convention is kept the same among the files using it

[c++] Unresolved external symbol in object files

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