Undefined reference to template class constructor

Question

I have no idea why this is happenning  since I think I have everything properly declared and defined   I have the following program  designed with templates  It s a simple implementation of a queue  with the member functions  add    substract  and  print    I have defined the node for the queue in the fine  nodo colaypila h     ifndef NODO COLAYPILA H  define NODO COLAYPILA H   include  lt iostream gt   template  lt class T gt  class cola   template  lt class T gt  class nodo colaypila           T elem          nodo colaypila lt T gt   sig          friend class cola lt T gt       public          nodo colaypila T  nodo colaypila lt T gt           Then the implementation in  nodo colaypila cpp    include  nodo colaypila h   include  lt iostream gt   template  lt class T gt  nodo colaypila lt T gt   nodo colaypila T a  nodo colaypila lt T gt   siguiente   NULL        elem   a      sig   siguiente   ctor     Afterwards  the definition and declaration of the queue template class and its functions    cola h     ifndef COLA H  define COLA H   include  nodo colaypila h   template  lt class T gt  class cola           nodo colaypila lt T gt   ult  pri      public          cola lt T gt             void anade T amp            T saca            void print   const          virtual  cola           endif    COLA H    cola cpp     include  cola h   include  nodo colaypila h    include  lt iostream gt   using namespace std   template  lt class T gt  cola lt T gt   cola         pri   NULL      ult   NULL   ctor    template  lt class T gt  void cola lt T gt   anade T amp  valor        nodo colaypila  lt T gt    nuevo       if  ult                nuevo   new nodo colaypila lt T gt   valor           ult- gt sig   nuevo          ult   nuevo            if   pri                pri   nuevo           template  lt class T gt  T cola lt T gt   saca         nodo colaypila  lt T gt    aux      T valor       aux   pri      if   aux                return 0            pri   aux- gt sig      valor   aux- gt elem      delete aux      if  pri                ult   NULL            return valor     template  lt class T gt  cola lt T gt    cola         while pri                saca           dtor    template  lt class T gt  void cola lt T gt   print   const       nodo colaypila  lt T gt    aux      aux   pri      while aux                cout  lt  lt  aux- gt elem  lt  lt  endl          aux   aux- gt sig            Then  I have a program to test these functions as follows    main cpp    include  lt iostream gt   include  cola h   include  nodo colaypila h   using namespace std   int main         float a  b  c      string d  e  f      cola lt float gt  flo      cola lt string gt  str       a   3 14      b   2 71      c   6 02      flo anade a       flo anade b       flo anade c       flo print        cout  lt  lt  endl       d    John       e    Mark       f    Matthew       str anade d       str anade e       str anade f       cout  lt  lt  endl       c   flo saca        cout  lt  lt   First In First Out Float     lt  lt  c  lt  lt  endl      cout  lt  lt  endl       f   str saca        cout  lt  lt   First In First Out String     lt  lt  f  lt  lt  endl      cout  lt  lt  endl       flo print        cout  lt  lt  endl      str print         cout  lt  lt   Hello world    lt  lt  endl      return 0      But when I build  the compiler throws errors in every instance of the template class    undefined reference to  cola float   cola        it s actually cola  lt  float     cola    but this doesn t let me use it like that    And so on  Altogether  17 warnings  counting the ones for the member functions being called in the program   Why is this  Those functions and constructors WERE defined  I thought that the compiler could replace the  T  in the template with  float    string  or whatever  that was the advantage of using templates   I read somewhere here that I should put the declaration of each function in the header file for some reason  Is that right  And if so  why   Thanks in advance

User · Answer

This link explains where you re going wrong    35 12  Why can t I separate the definition of my templates class from its declaration and put it inside a  cpp file   Place the definition of your constructors  destructors methods and whatnot in your header file  and that will correct the problem   This offers another solution   How can I avoid linker errors with my template functions   However this requires you to anticipate how your template will be used and  as a general solution  is counter-intuitive  It does solve the corner case though where you develop a template to be used by some internal mechanism  and you want to police the manner in which it is used

User · Answer

This is a common question in C   programming  There are two valid answers to this  There are advantages and disadvantages to both answers and your choice will depend on context  The common answer is to put all the implementation in the header file  but there s another approach will will be suitable in some cases  The choice is yours   The code in a template is merely a  pattern  known to the compiler  The compiler won t compile the constructors cola lt float gt   cola      and cola lt string gt   cola      until it is forced to do so  And we must ensure that this compilation happens for the constructors at least once in the entire compilation process  or we will get the  undefined reference  error   This applies to the other methods of cola lt T gt  also    Understanding the problem  The problem is caused by the fact that main cpp and cola cpp will be compiled separately first  In main cpp  the compiler will implicitly instantiate the template classes cola lt float gt  and cola lt string gt  because those particular instantiations are used in main cpp  The bad news is that the implementations of those member functions are not in main cpp  nor in any header file included in main cpp  and therefore the compiler can t include complete versions of those functions in main o  When compiling cola cpp  the compiler won t compile those instantiations either  because there are no implicit or explicit instantiations of cola lt float gt  or cola lt string gt   Remember  when compiling cola cpp  the compiler has no clue which instantiations will be needed  and we can t expect it to compile for every type in order to ensure this problem never happens   cola lt int gt   cola lt char gt   cola lt ostream gt   cola lt  cola lt int gt   gt      and so on       The two answers are    Tell the compiler  at the end of cola cpp  which particular template classes will be required  forcing it to compile cola lt float gt  and cola lt string gt   Put the implementation of the member functions in a header file that will be included every time any other  translation unit   such as main cpp  uses the template class    Answer 1  Explicitly instantiate the template  and its member definitions  At the end of cola cpp  you should add lines explicitly instantiating all the relevant templates  such as  template class cola lt float gt   template class cola lt string gt     and you add the following two lines at the end of nodo colaypila cpp   template class nodo colaypila lt float gt   template class nodo colaypila lt std    string gt     This will ensure that  when the compiler is compiling cola cpp that it will explicitly compile all the code for the cola lt float gt  and cola lt string gt  classes  Similarly  nodo colaypila cpp contains the implementations of the nodo colaypila lt     gt  classes   In this approach  you should ensure that all the of the implementation is placed into one  cpp file  i e  one translation unit  and that the explicit instantation is placed after the definition of all the functions  i e  at the end of the file    Answer 2  Copy the code into the relevant header file  The common answer is to move all the code from the implementation files cola cpp and nodo colaypila cpp into cola h and nodo colaypila h  In the long run  this is more flexible as it means you can use extra instantiations  e g  cola lt char gt   without any more work  But it could mean the same functions are compiled many times  once in each translation unit  This is not a big problem  as the linker will correctly ignore the duplicate implementations  But it might slow down the compilation a little   Summary  The default answer  used by the STL for example and in most of the code that any of us will write  is to put all the implementations in the header files  But in a more private project  you will have more knowledge and control of which particular template classes will be instantiated  In fact  this  bug  might be seen as a feature  as it stops users of your code from accidentally using instantiations you have not tested for or planned for   I know this works for cola lt float gt  and cola lt string gt   if you want to use something else  tell me first and will can verify it works before enabling it      Finally  there are three other minor typos in the code in your question    You are missing an  endif at the end of nodo colaypila h in cola h nodo colaypila lt T gt   ult  pri  should be nodo colaypila lt T gt   ult   pri  - both are pointers  nodo colaypila cpp  The default parameter should be in the header file nodo colaypila h  not in this implementation file

User · Answer

You will have to define the functions inside your header file  You cannot separate definition of template functions in to the source file and declarations in to header file       When a template is used in a way that triggers its intstantation  a compiler needs to see that particular templates definition  This is the reason templates are often defined in the header file in which they are declared     Reference  C  03 standard     14 7 2 4      The definition of a non-exported function template  a non-exported member function template  or a non-exported member function or static data member of a class template shall be present in every translation unit in which it is explicitly instantiated    EDIT  To clarify the discussion on the comments  Technically  there are three ways to get around this linking problem    To move the definition to the  h file  Add explicit instantiations in the  cpp file    include the  cpp file defining the template at the  cpp file using the template     Each of them have their pros and cons       Moving the defintions to header files may increase the code size modern day compilers can avoid this  but will increase the compilation time for sure       Using the explicit instantiation approach is moving back on to traditional macro like approach Another disadvantage is that it is necessary to know which template types are needed by the program  For a simple program this is easy but for complicated program this becomes difficult to determine in advance            While including cpp files is confusing at the same time shares the problems of both above approaches   I find first method the easiest to follow and implement and hence advocte using it

[c++] "Undefined reference to" template class constructor

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