"Undefined Reference to..." in Template Class

[Philip Lee Bridson]

Good Evening,

I have two template classes, cut down versions are shown below:

// ClassA.hpp
#ifndef _ClassA_hpp_
#define _ClassA_hpp_

#include <iostream>
#include <vector>

template< class _Tp >
class A
{
     private: std::vector< _Tp > m_value;
     public: void add( const _Tp& value )
     {
          m_value.push_back(value);
     }
};

#endif

// ClassB.hpp
#ifndef _ClassB_hpp_
#define _ClassB_hpp_

#include "ClassA.hpp"

template< class _Tp >
class B
{
     private: static ClassA< B* > m_value;
     public: B()
     {
          m_value.add( this );
     }
};

#endif 

Class B is instantiated as:

ClassB< int >* = new ClassB< int >();

But I get the error: undefined reference to ClassB< int >::m_value in every method that references m_value. Most the examples I could on the net of this error relate to users who seperate the implementation out of the template file. I have not done this though. So the only reason I could come up with is that it might be something to do with the template instantiation such as this would fail:

ClassB< int >* = new ClassB< int >();
ClassB< std::string >* = new ClassB< std::string >(); // this would fail maybe? 

I guess my question is - am I right and if so how do I get around the issue?

Thank you for any assistance offered.

Phil.

[firedraco]

I wasn't getting that error. I was getting different errors, which first I had to fix several of your syntax errors in that post (ClassA should be A, etc). Once I did that, the error turned out to be that the variable wasn't defined anywhere (as statics should be), and once I defined it, the program compiled fine.

//the other stuff goes above
int main() {
	B< int >* temp = new B< int >;
}

[Philip Lee Bridson]

Thanks for the reply, I have just re-read my post and am appalled at how many mistakes were in it. Unfortunately they were just typos as I am not permitted to post actual code on here which would make things a whole lot easier. Class B implements a wrapper for a pointer, Class A implements a list. Both are templates declared and defined in their own files. There are no syntax errors in the actual files and the errors only happen when linking.

[Philip Lee Bridson]

Good Evening,

Sorry about the post last night I was trying to post an article off my phone which obviously failed. I have conducted some tests today and essentially confirmed what I initially believed which is it is because of the template initialization. Essentially what I need to do is this:

#include <iostream>

template< class _Tp >
class A
{};

template< class _Tp >
class B
{};

int main (int argc, char * const argv[]) {
	B< A* >* b = new B< A* >();     // this complains that A is missing template parameter
    return 0;
}     

So I need to be able to add a pointer to A to the list no matter what template type A has. Is this possible at all? GCC seems to complain about each different method I have tried.

Many thanks,

Phil.

[ne555]

¿so you want an heterogeneous container? ¿what for?
By the way B<int> temp; that is how an object is created. (in this case using the default constructor)

[Philip Lee Bridson]

Class B maintains a list of all instance's of class A that are created. This would not be so much of a problem if Class A were not a template but it is and I can't do much about that. So somehow I need to figure out how to add any instance of class A to class B such that I can do this:

A<int>* a = new A<int>(0);     // assume there is a constructor to do this
A<string>* b = new A<string>("foo");     // and for this
// etc... for any type or custom class you like....

B< A* >* d = new B< A* >();     // creates a new list
d->add(a);
d->add(b);

// and on and on....
// for ANY type of A that is created.

That's what I'd like to do but it complains about a missing template parameter when creating the instance of B.

Thank you for your response so far.

Phil.