Linker problem with templated function

[meLlamanJefe]

The following is an abridged copy of the code I am using in a template function along with the linker error produced during the build. The compiler has no issues with the code, only the linker:

file DB.h

 
enum TypeEnum
   {
      eDouble,
      eInteger,
      eBool,
      eString,
      eFloat,
      eLong,
      TypeEnumRange
   };
 
template <class T>
class DataBusGenericOutputType
{
      public:
 
         DataBusGenericOutputType();
 
         void    SetValue(T data){ GT->SetValue(data); } ;
 
      private:
 
         DataBusGenericType<T>* GT;
};
 
class DataBus
{
   public:
 
   ......
 
      template <typename T> DataBusGenericOutputType<T> *Register2(std::string ID, TypeEnum type, T value);
 
   ......
 
   private:
 
   ......
 
        std::list< DataBusGenericType<double> >  DoubleList;
        std::list< DataBusGenericType<string> >  StringList;
        std::list< DataBusGenericType<int>    >  IntList;
        std::list< DataBusGenericType<bool>   >  BoolList;
        std::list< DataBusGenericType<float>  >  FloatList;
        std::list< DataBusGenericType<long>   >  LongList;
 
   ......
};
 

file DB.cpp

template <typename T> 
DataBusGenericOutputType<T> *DataBus::Register2(std::string ID, TypeEnum type, T value)
{
   DataBusGenericOutputType<T> *OutType = NULL;
   DataBusGenericType<T>       *GenType = NULL;
   std::vector<string> IDparts;
 
   IDparts = ParseID(ID); 
 
   GenType = new DataBusGenericType<T>(IDparts[ClassLocation], IDparts[VariableLocation], IDparts[TypeLocation], type, value);
 
   switch(type)
   {
      case eDouble:
           DoubleList.push_back(*GenType);
           OutType = new DataBusGenericOutputType<T>(&DoubleList.back());
           break;
 
      case eInteger:
           IntList.push_back(*GenType);
           OutType = new DataBusGenericOutputType<T>(&IntList.back());
           break;
 
      case eBool:
           BoolList.push_back(*GenType);
           OutType = new DataBusGenericOutputType<T>(&BoolList.back());
           break;
 
      case eString:
           StringList.push_back(*GenType);
           OutType = new DataBusGenericOutputType<T>(&StringList.back());
           break;
 
      case eFloat:
           FloatList.push_back(*GenType);
           OutType = new DataBusGenericOutputType<T>(&FloatList.back());
           break;
 
      case eLong:
           LongList.push_back(*GenType);
           OutType = new DataBusGenericOutputType<T>(&LongList.back());
           break;
 
      default: 
           break;
 
   }
   return OutType;
}
 

file Dummy.cpp

#include "DB.h"
 
void Dummy::Register()
{
   DataBus *DB = new DataBus();
   double mDummyDouble;
   DataBusGenericOutputType<double> *oDummyDouble;
 
oDummyDouble = DB->Register2("Dummy.DummyDouble.double", eDouble, mDummyDouble); // <---This is line 115 which the linker error references
 
....
}
 

Error from the linker:
./Lib/libBigBastard.a(Dummy.o)(.text+0x6e6): In function `Dummy::Register()':
/home/jr/Dummy.cpp:115: undefined reference to `DataBusGenericOutputType<double>* DataBus::Register2<double>(std::basic_string<char, std::char_traits<char>, std::allocator<char> >, TypeEnum, double)'

Again, the compiler gives no errors so all local and stl headers have been properly included and all functions and types seem to be accessible by the code. For what it's worth, the code is being built using gcc version 3.2.2.

Thanks!

[Savage]

That function is not a class method.It's outside of DataBusGenericOutputType class scope..

[meLlamanJefe]

That function is not a class method.It's outside of DataBusGenericOutputType class scope..

I apologize for that, in my rush to post the question I did not clearly specify the class declaration. Please see the edited original post again.

Thanks!

[Savage]

I apologize for that, in my rush to post the question I did not clearly specify the class declaration. Please see the edited original post again.

Thanks!

Template functions must be defined in the same file where they are declared.

NOTE:

There are ways around it like using export keyword or including a cpp file with function definitions just after template class declarations,but they can really complicate your life.Export templates keyword is not supported in all compilers and including a definition cpp file can create mess in your project organization.

[meLlamanJefe]

Template functions must be defined in the same file where they are declared.

NOTE:

There are ways around it like using export keyword or including a cpp file with function definitions just after template class declarations,but they can really complicate your life.Export templates keyword is not supported in all compilers and including a definition cpp file can create mess in your project organization.

Thanks! moving the definition did help. However, it turns out that the compiler doesn't like the use of the template type inside the switch statements so this code is not good anyway... On to a different approach.

[weaknessforcats]

There are ways around it like using export keyword

Quick tell me the name of any compiler that supports this. I have been telling my students there are no such compilers. Maybe I need an update.

[Savage]

Quick tell me the name of any compiler that supports this. I have been telling my students there are no such compilers. Maybe I need an update.

I'm not sure if Intel's 7.x compiler does support it,but I'm sure Comeau does.

Not very wide support indeed, making export for templates pretty useless.

[oler1s]

Not very wide support indeed, making export for templates pretty useless.

Support or not, export for templates is very useless. Read up on Google on the problem with export. It does not work like you think it does.