failing to invoke template class, c++

Question

I've defined a template class like so (providing .hpp file):

#ifndef PERSOANLVEC_H_
#define PERSOANLVEC_H_

#include <vector>

using namespace std;

template<class T, class PrnT> class PersoanlVec {
public:
    PersoanlVec();
    ~PersoanlVec();
    void push_back(T t);
    void erase(int index);
    PersoanlVec& operator[](int index);
    const PersoanlVec& operator[](int index) const;
    void print() const;
    size_t size();
private:
    vector<T> _vector;
};

#endif /* PERSOANLVEC_H_ */

Now, everything compiles ok with this class. When I try to use it I get undefined reference to PersoanlVec<Person, Person>::PersoanlVec()'. Here's where I call it:

#include "Person.h"
#include "PersoanlVec.hpp"
#include <cstdlib>

int main(void)
{
    Person p1("yotam");
    Person p2("yaara");
    PersoanlVec<Person,Person> *a = new PersoanlVec<Person,Person>(); //<---ERROR HERE
    return EXIT_SUCCESS;
}

This is my first try with templates, its not very clear for me obviously. I DO have a constructor with no parameters, Any ideas? Thanks!

where's implementation of `PersoanlVec`? (btw, was it intended to be `PersonalVec`?) — Andriy Tylychko, Sep 19 '11 at 13:00

score 4 · Answer 1 · edited May 23 '17 at 12:12

Do you have the content of your constructor and functions in a .cpp file? If yes, there's your problem. Put them in the header file, possible just inline in the class itself:

template<class T, class PrnT> class PersoanlVec {
public:
    PersoanlVec(){
      // code here...
    }

    ~PersoanlVec(){
      // code here...
    }

    void push_back(T t){
      // code here...
    }

    void erase(int index){
      // code here...
    }

    PersoanlVec& operator[](int index){
      // code here...
    }

    const PersoanlVec& operator[](int index) const{
      // code here...
    }

    void print() const{
      // code here...
    }

    size_t size(){
      // code here...
    }

private:
    vector<T> _vector;
};

For the reason, have a look here.

score 0 · Answer 2 · answered Sep 19 '11 at 13:08

I am pretty sure you just forgot to add the file to the compilation process. Be careful with your misspelling, since that can cause generic pain.

Whenever you have different compilation units (classes for example, each with their .h/.cpp), your classes need to know of the interfaces, reason for which you normally include the header files, yet the compiler also needs to know the implementations so that it can bind together your binary file.

As such, you will need to call the compiler passing all the .cpp files in your project to it, otherwise it will fail letting you know you are referencing unimplemented pieces.

Though this is the case for regular classes, class templates don't follow that rule. For the reason, see w00te's or my answer. — Xeo, Sep 19 '11 at 13:18

score 0 · Answer 3 · answered Sep 19 '11 at 13:14

You need to have all of your template function definitions held in the header file rather than the CPP file - this is basically because the template definition will be used multiple times to create multiple types depending on what parameters you pass in to it as type parameters around your code. The only template related functions that should ever be defined in the CPP file are template specialization functions - those where you want to explicitly say (if user passed in type A and B then do this specifically instead of the default action).

failing to invoke template class, c++

3 Answers3