Question

I am currently working in Ubuntu9.10 – c++.
I need to define an generic object in a method. I have to define the method in .h file. How can I do it? I did the following:

file.h

class ana

{
//code
public:
template <class T>  bool method (T &Data);
};

file.cpp

//code

template <class T>
bool ana::method(T &Data)
{
//code
}

I’ve created the .a file.

In test.cpp:

//code
main()
{
    //code
    Ana* ann = new Ana();

    if (ann->method(*ann)){//code}
}

After compiling with g++ test.cpp -o test libfile.a I have the error: undefined reference to bool.... Why? Is there another way to create a generic object?

Answer 1

Putting function definitions together with declarations (in the header files itself) definitely helps and is the way of choice. But, in case you want to separate the function definitions, have the following lines at the end of the .cpp file. Its called explicit instantiation.This will take care of linker errors. I tried some code, and this seems to work, based on what you have given:

file.h:

#ifndef __ANA_H__
#define __ANA_H__

template <class T>
class ana {

  public: 
    bool method(T& data);
};

#endif

file.cpp:

#include <ana.h>
#include <iostream>
using namespace std;

template <typename T>
bool ana<T>::method(T& data) {
  cout << "Data = " << data << endl;
  if(data > 0) {
    return true;
  }
  return false;
}

//explicit instantiation for avoidance of g++ linker errors.
template
bool ana<int>::method(int& data);

template
bool ana<double>::method(double& data)

One of the downsides of using this method is that these lines will have to be included for every data type that you want this function to support. So, now the method function will run ONLY for int and double. The specs for your code should be such that method is never called for data types other than the above.
HTH,
Sriram