Undefined reference to vtable

Question

When building my C   program  I m getting the error message      undefined reference to  vtable      What is the cause of this problem  How do I fix it     It so happens that I m getting the error for the following code  The class in question is CGameModule   and I cannot for the life of me understand what the problem is  At first  I thought it was related to forgetting to give a virtual function a body  but as far as I understand  everything is all here  The inheritance chain is a little long  but here is the related source code  I m not sure what other information I should provide   Note  The constructor is where this error is happening  it d seem   My code   class CGameModule   public CDasherModule    public    CGameModule Dasher  CEventHandler  pEventHandler  CSettingsStore  pSettingsStore  CDasherInterfaceBase  pInterface  ModuleID t iID  const char  szName      CDasherModule pEventHandler  pSettingsStore  iID  0  szName             g pLogger- gt Log  Inside game module constructor             m pInterface   pInterface          virtual  CGameModule          std  string GetTypedTarget       std  string GetUntypedTarget       bool DecorateView CDasherView  pView            g pLogger- gt Log  Decorating the view          return false         void SetDasherModel CDasherModel  pModel    m pModel   pModel        virtual void HandleEvent Dasher  CEvent  pEvent      private       CDasherNode  pLastTypedNode      CDasherNode  pNextTargetNode      std  string m sTargetString      size t m stCurrentStringPos      CDasherModel  m pModel      CDasherInterfaceBase  m pInterface       Inherits from     class CDasherModule  typedef std  vector lt CDasherModule  gt   size type ModuleID t        ingroup Core        class CDasherModule   public Dasher  CDasherComponent    public    CDasherModule Dasher  CEventHandler   pEventHandler  CSettingsStore   pSettingsStore  ModuleID t iID  int iType  const char  szName      virtual ModuleID t GetID      virtual void SetID ModuleID t     virtual int GetType      virtual const char  GetName       virtual bool GetSettings SModuleSettings   pSettings  int  iCount        return false         private    ModuleID t m iID    int m iType    const char  m szName       Which inherits from      namespace Dasher     class CEvent    class CEventHandler    class CDasherComponent           ingroup Core        class Dasher  CDasherComponent    public    CDasherComponent Dasher  CEventHandler  pEventHandler  CSettingsStore  pSettingsStore     virtual  CDasherComponent       void InsertEvent Dasher  CEvent   pEvent     virtual void HandleEvent Dasher  CEvent   pEvent         bool GetBoolParameter int iParameter  const    void SetBoolParameter int iParameter  bool bValue  const     long GetLongParameter int iParameter  const    void SetLongParameter int iParameter  long lValue  const     std  string GetStringParameter int iParameter  const    void        SetStringParameter int iParameter  const std  string  amp  sValue  const     ParameterType   GetParameterType int iParameter  const    std  string     GetParameterName int iParameter  const    protected    Dasher  CEventHandler  m pEventHandler    CSettingsStore  m pSettingsStore               endif

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Not to cross post but  If you are dealing with inheritance the second google hit was what I had missed  ie  all virtual methods should be defined   Such as   virtual void fooBar     0    See answare C   Undefined Reference to vtable and inheritance for details  Just realized it s already mentioned above  but heck it might help someone

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In my case I m using Qt and had defined a QObject subclass in a foo cpp  not  h  file  The fix was to add  include  foo moc  at the end of foo cpp

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If you are using Qt  try rerunning qmake  If this error is in the widget s class  qmake might have failed to notice that the ui class vtable should be regenerated  This fixed the issue for me

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I think it s also worth mentioning that you will also get the message when you try to link to object of any class that has at least one virtual method and linker cannot find the file  For example   Foo hpp   class Foo   public      virtual void StartFooing         Foo cpp    include  Foo hpp   void Foo  StartFooing      fooing     Compiled with   g   Foo cpp -c   And main cpp    include  Foo hpp   int main         Foo foo      Compiled and linked with   g   main cpp -o main   Gives our favourite error       tmp cclKnW0g o  In function main   main cpp   text 0x1a   undefined   reference tovtable for Foo  collect2  error  ld returned 1 exit   status   This occure from my undestanding becasue    Vtable is created per class at compile time Linker does not have access to vtable that is in Foo o

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For what it is worth  forgetting a body on a virtual destructor generates the following       undefined reference to  vtable for CYourClass       I am adding a note because the error message is deceptive    This was with gcc version 4 6 3

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The GCC FAQ has an entry on it   The solution is to ensure that all virtual methods that are not pure are defined  Note that a destructor must be defined even if it is declared pure-virtual  class dtor  7   Therefore  you need to provide a definition for the virtual destructor  virtual  CDasherModule

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I tried all the detailed steps by JaMIT and still got stumped by this error  After a good amount of head-banging  I figured it out  I was careless  You should be able to reproduce this painful-to-look-at error w  the following sample code   jaswantp jaswant-arch build   gcc -v Using built-in specs  COLLECT GCC gcc COLLECT LTO WRAPPER  usr lib gcc x86 64-pc-linux-gnu 10 2 0 lto-wrapper Target  x86 64-pc-linux-gnu Configured with   build gcc src gcc configure --prefix  usr --libdir  usr lib --libexecdir  usr lib --mandir  usr share man --infodir  usr share info --with-bugurl https   bugs archlinux org  --enable-languages c c   ada fortran go lto objc obj-c   d --with-isl --with-linker-hash-style gnu --with-system-zlib --enable-  cxa atexit --enable-cet auto --enable-checking release --enable-clocale gnu --enable-default-pie --enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object --enable-install-libiberty --enable-linker-build-id --enable-lto --enable-multilib --enable-plugin --enable-shared --enable-threads posix --disable-libssp --disable-libstdcxx-pch --disable-libunwind-exceptions --disable-werror gdc include dir  usr include dlang gdc Thread model  posix Supported LTO compression algorithms  zlib zstd gcc version 10 2 0  GCC        CelesetialBody h class CelestialBody  public      virtual void Print    protected      CelestialBody        virtual  CelestialBody           CelestialBody cpp  include  quot CelestialBody h quot   CelestialBody  CelestialBody       CelestialBody   CelestialBody     default   void CelestialBody  Print          Planet h  include  quot CelestialBody h quot   class Planet   public CelestialBody   public      void Print   override  protected      Planet         Planet   override         Planet cpp  include  quot Planet h quot   Planet  Planet      Planet   Planet       void Print         Deliberately forgot to prefix  Planet       CMakeLists txt cmake minimum required VERSION 3 12  project  space engine  add library  CelestialBody SHARED CelestialBody cpp  add library  Planet SHARED Planet cpp  target include directories  CelestialBody PRIVATE   CMAKE CURRENT LIST DIR     target include directories  Planet PRIVATE   CMAKE CURRENT LIST DIR       target link libraries  Planet PUBLIC CelestialBody     hardened linker flags to catch undefined symbols target link options Planet      PRIVATE      -Wl --as-needed     -Wl --no-undefined    And we get our favourite error    mkdir build   cd build   cmake      make   50   Built target CelestialBody Scanning dependencies of target Planet   75   Building CXX object CMakeFiles Planet dir Planet cpp o  100   Linking CXX shared library libPlanet so  usr bin ld  CMakeFiles Planet dir Planet cpp o  in function  Planet  Planet     Planet cpp   text 0x1b   undefined reference to  vtable for Planet   usr bin ld  CMakeFiles Planet dir Planet cpp o  in function  Planet   Planet     Planet cpp   text 0x3d   undefined reference to  vtable for Planet  collect2  error  ld returned 1 exit status make 2        CMakeFiles Planet dir build make 104  libPlanet so  Error 1 make 1        CMakeFiles Makefile2 97  CMakeFiles Planet dir all  Error 2 make       Makefile 103  all  Error 2   What I ve done in Planet cpp should of course be resolved with this tip   Look at your class definition  Find the first non-inline virtual function that is not pure virtual  not  quot   0 quot   and whose definition you provide  not  quot   default quot      from JaMIT s answer  If there is anyone else who tried all the above and nothing worked  maybe you too  like me  carelessly forgot to prefix  lt ClassName gt    to one or more member functions  Either I need to get my eyes checked or I need to get some sleep

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Undefined reference to vtable may occur due to the following situation also  Just try this   Class A Contains   virtual void functionA parameters  0   virtual void functionB parameters     Class B Contains    The definition for the above functionA   The definition for the above functionB    Class C Contains  Now you re writing a Class C in which you are going to derive it from Class A    Now if you try to compile you will get Undefined reference to vtable for Class C as error   Reason   functionA is defined as pure virtual and its definition is provided in Class B  functionB is defined as virtual  NOT PURE VIRTUAL  so it tries to find its definition in Class A itself but you provided its definition in Class B   Solution    Make function B as pure virtual  if you have requirement like that  virtual void functionB parameters   0   This works it is Tested  Provide Definition for functionB in Class A itself keeping it as virtual    Hope it works as I didn t try this

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The GNU C   compiler has to make a decision where to put the vtable in case you have the definition of the virtual functions of an object spread across multiple compilations units  e g  some of the objects virtual functions definitions are in a  cpp file others in another  cpp file  and so on     The compiler chooses to put the vtable in the same place as where the first declared virtual function is defined   Now if you for some reason forgot to provide a definition for that first virtual function declared in the object  or mistakenly forgot to add the compiled object at linking phase   you will get this error    As a side effect  please note that only for this particular virtual function you won t get the traditional linker error like you are missing function foo

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What is a vtable   It might be useful to know what the error message is talking about before trying to fix it  I ll start at a high level  then work down to some more details  That way people can skip ahead once they are comfortable with their understanding of vtables   hellip and there goes a bunch of people skipping ahead right now     For those sticking around   A vtable is basically the most common implementation of polymorphism in C    When vtables are used  every polymorphic class has a vtable somewhere in the program  you can think of it as a  hidden  static data member of the class  Every object of a polymorphic class is associated with the vtable for its most-derived class  By checking this association  the program can work its polymorphic magic  Important caveat  a vtable is an implementation detail  It is not mandated by the C   standard  even though most  all   C   compilers use vtables to implement polymorphic behavior  The details I am presenting are either typical or reasonable approaches  Compilers are allowed to deviate from this   Each polymorphic object has a  hidden  pointer to the vtable for the object s most-derived class  possibly multiple pointers  in the more complex cases   By looking at the pointer  the program can tell what the  real  type of an object is  except during construction  but let s skip that special case   For example  if an object of type A does not point to the vtable of A  then that object is actually a sub-object of something derived from A   The name  vtable  comes from  virtual function table   It is a table that stores pointers to  virtual  functions  A compiler chooses its convention for how the table is laid out  a simple approach is to go through the virtual functions in the order they are declared within class definitions  When a virtual function is called  the program follows the object s pointer to a vtable  goes to the entry associated with the desired function  then uses the stored function pointer to invoke the correct function  There are various tricks for making this work  but I won t go into those here   Where when is a vtable generated   A vtable is automatically generated  sometimes called  emitted   by the compiler  A compiler could emit a vtable in every translation unit that sees a polymorphic class definition  but that would usually be unnecessary overkill  An alternative  used by gcc  and probably by others  is to pick a single translation unit in which to place the vtable  similar to how you would pick a single source file in which to put a class  static data members  If this selection process fails to pick any translation units  then the vtable becomes an undefined reference  Hence the error  whose message is admittedly not particularly clear   Similarly  if the selection process does pick a translation unit  but that object file is not provided to the linker  then the vtable becomes an undefined reference  Unfortunately  the error message can be even less clear in this case than in the case where the selection process failed   Thanks to the answerers who mentioned this possibility  I probably would have forgotten it otherwise    The selection process used by gcc makes sense if we start with the tradition of devoting a  single  source file to each class that needs one for its implementation  It would be nice to emit the vtable when compiling that source file  Let s call that our goal  However  the selection process needs to work even if this tradition is not followed  So instead of looking for the implementation of the entire class  let s look for the implementation of a specific member of the class  If tradition is followed  ndash  and if that member is in fact implemented  ndash  then this achieves the goal   The member selected by gcc  and potentially by other compilers  is the first non-inline virtual function that is not pure virtual  If you are part of the crowd that declares constructors and destructors before other member functions  then that destructor has a good chance of being selected   You did remember to make the destructor virtual  right   There are exceptions  I d expect that the most common exceptions are when an inline definition is provided for the destructor and when the default destructor is requested  using    default     The astute might notice that a polymorphic class is allowed to provide inline definitions for all of its virtual functions  Doesn t that cause the selection process to fail  It does in older compilers  I ve read that the latest compilers have addressed this situation  but I do not know relevant version numbers  I could try looking this up  but it s easier to either code around it or wait for the compiler to complain   In summary  there are three key causes of the  undefined reference to vtable  error    A member function is missing its definition  An object file is not being linked  All virtual functions have inline definitions    These causes are by themselves insufficient to cause the error on their own  Rather  these are what you would address to resolve the error  Do not expect that intentionally creating one of these situations will definitely produce this error  there are other requirements  Do expect that resolving these situations will resolve this error    OK  number 3 might have been sufficient when this question was asked    How to fix the error   Welcome back people skipping ahead       Look at your class definition  Find the first non-inline virtual function that is not pure virtual  not    0   and whose definition you provide  not    default      If there is no such function  try modifying your class so there is one   Error possibly resolved   See also the answer by Philip Thomas for a caveat   Find the definition for that function  If it is missing  add it   Error possibly resolved     See also the answer by RedSpikeyThing for a caveat   Check your link command  If it does not mention the object file with that function s definition  fix that   Error possibly resolved   Repeat steps 2 and 3 for each virtual function  then for each non-virtual function  until the error is resolved  If you re still stuck  repeat for each static data member    Example The details of what to do can vary  and sometimes branch off into separate questions  like What is an undefined reference unresolved external symbol error and how do I fix it    I will  though  provide an example of what to do in a specific case that might befuddle newer programmers   Step 1 mentions modifying your class so that it has a function of a certain type  If the description of that function went over your head  you might be in the situation I intend to address  Keep in mind that this is a way to accomplish the goal  it is not the only way  and there easily could be better ways in your specific situation  Let s call your class A  Is your destructor declared  in your class definition  as either  virtual  A     default    or  virtual  A          If so  two steps will change your destructor into the type of function we want  First  change that line to  virtual  A      Second  put the following line in a source file that is part of your project  preferably the file with the class implementation  if you have one    A   A        That makes your  virtual  destructor non-inline and not generated by the compiler   Feel free to modify things to better match your code formatting style  such as adding a header comment to the function definition

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I simply got this error because my  cpp file was not in the makefile  In general  if you forget to compile or link to the specific object file containing the definition  you will run into this error

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So I was using Qt with Windows XP and MinGW compiler and this thing was driving me crazy   Basically the moc xxx cpp was generated empty even when I was added  Q OBJECT  Deleting everything making functions virtual  explicit and whatever you guess doesn t worked  Finally I started removing line by line and it turned out that I had   ifdef something   Around the file  Even when the  ifdef was true moc file was not generated   So removing all  ifdefs fixed the problem   This thing was not happening with Windows and VS 2013

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So  I ve figured out the issue and it was a combination of bad logic and not being totally familiar with the automake autotools world  I was adding the correct files to my Makefile am template  but I wasn t sure which step in our build process actually created the makefile itself  So  I was compiling with an old makefile that had no idea about my new files whatsoever   Thanks for the responses and the link to the GCC FAQ  I will be sure to read that to avoid this problem occurring for a real reason

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I just ran into another cause for this error that you can check for   The base class defined a pure virtual function as   virtual int foo int x   0     And the subclass had  int foo int x  override    The problem was the typo that the   0  was supposed to be outside of the parenthesis   virtual int foo int x    0    So  in case you re scrolling this far down  you probably didn t find the answer - this is something else to check for

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Are you sure that CDasherComponent has a body for the destructor    It s definitely not here - the question is if it is in the  cc file  From a style perspective  CDasherModule should explicitly define its destructor virtual  It looks like CGameModule has an extra    at the end  after the       for the class   Is CGameModule being linked against the libraries that define CDasherModule and CDasherComponent

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If all else fails  look for duplication   I was misdirected by the explicit initial reference to constructors and destructors until I read a reference in another post   It s any unresolved method   In my case  I thought I had replaced the declaration that used char  xml as the parameter with one using the unnecessarily troublesome const char  xml  but instead  I had created a new one and left the other one in place

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There is lot of speculation going on in various answers here  I ll below give a fairly minimal code that reproduces this error and explain why it is occuring    Fairly Minimal Code to Reproduce This Error  IBase hpp   pragma once  class IBase       public          virtual void action     0       Derived hpp   pragma once   include  IBase hpp   class Derived   public IBase       public          Derived int a           void action   override       Derived cpp   include  Derived hpp  Derived  Derived int a      void Derived  action        myclass cpp   include  lt memory gt   include  Derived hpp   class MyClass        public          MyClass std  shared ptr lt Derived gt  newInstance    instance newInstance                        void doSomething                 instance- gt action                   private          std  shared ptr lt Derived gt  instance      int main int argc  char   argv        Derived myInstance 5       MyClass c std  make shared lt Derived gt  myInstance        c doSomething        return 0      You can compile this using GCC like this   g   -std c  11 -o a out myclass cpp Derived cpp   You can now reproduce the error by removing    0 in IBase hpp  I get this error         catkin ws  g   -std c  11 -o  tmp m out  tmp myclass cpp  tmp Derived cpp  tmp cclLscB9 o  In function  IBase  IBase IBase const amp     myclass cpp   text  ZN5IBaseC2ERKS   ZN5IBaseC5ERKS   0x13   undefined reference to  vtable for IBase   tmp cc8Smvhm o  In function  IBase  IBase     Derived cpp   text  ZN5IBaseC2Ev  ZN5IBaseC5Ev  0xf   undefined reference to  vtable for IBase   tmp cc8Smvhm o   rodata  ZTI7Derived  ZTI7Derived  0x10   undefined reference to  typeinfo for IBase  collect2  error  ld returned 1 exit status   Explanation  Notice that above code does not require any virtual destructors  constructors or any other extra files for compile to be successful  although you should have them     The way to understand this error is as follows  Linker is looking for constructor of IBase  This it will need it for the constructor of Derived  However as Derived overrides methods from IBase  it has vtable attached to it that will reference IBase  When linker says  undefined reference to vtable for IBase  it basically means that Derived has vtable reference to IBase but it can t find any compiled object code of IBase to look up to  So the bottom line is that class IBase has declarations without implementations  This means a method in IBase is declared as virtual but we forgot to mark it as pure virtual OR provide its definition   Parting Tip  If all else fails then one way to debug this error is to build minimal program that does compile and then keep changing it so it gets to the state you want  In between  keep compiling to see when it starts to fail   Note on ROS and Catkin build system  If you were compiling above set of classes in ROS using catkin build system then you will need following lines in CMakeLists txt   add executable myclass src myclass cpp src Derived cpp  add dependencies myclass theseus myclass cpp  target link libraries myclass   catkin LIBRARIES     The first line basically says that we want to make an executable named myclass and the code to build this can be found files that follows  One of these files should have main    Notice that you don t have to specify  hpp files anywhere in CMakeLists txt  Also you don t have to specify Derived cpp as library

[c++] Undefined reference to vtable

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