"undefined reference to" linking on Ubuntu - c++

Question

I'm relatively new to C & C++ and stuck at compiling (or should I say linking) for the whole 2 days. Anyone gives me an idea would be appreciated.
Error message and 3 code files are below. These are what I cut down to minimum from I'm actually working on so that you guys can take a better glimpse at.
Env: Ubuntu 10.10, Eclipse Indigo CDT, g++ (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5
Error message:
**** Build of configuration Debug for project SceneRec2 ****
make all
Building file: ../src/AdaBoost.cpp
Invoking: GCC C++ Compiler
g++ -I"/home/ubuntuLove/Documents/workspace_eclipse/SceneRec2/Includes" -I/usr/src/linux-headers-2.6.35-30/arch/um/include/shared -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/AdaBoost.d" -MT"src/AdaBoost.d" -o "src/AdaBoost.o" "../src/AdaBoost.cpp"
Finished building: ../src/AdaBoost.cpp
Building file: ../src/AdaMain.cpp
Invoking: GCC C++ Compiler
g++ -I"/home/ubuntuLove/Documents/workspace_eclipse/SceneRec2/Includes" -I/usr/src/linux-headers-2.6.35-30/arch/um/include/shared -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/AdaMain.d" -MT"src/AdaMain.d" -o "src/AdaMain.o" "../src/AdaMain.cpp"
../src/AdaMain.cpp: In function ‘int main(int, char**)’:
../src/AdaMain.cpp:6: warning: deprecated conversion from string constant to ‘char*’
Finished building: ../src/AdaMain.cpp
Building target: SceneRec2
Invoking: GCC C++ Linker
g++ -o "SceneRec2" ./src/AdaBoost.o ./src/AdaMain.o
./src/AdaMain.o: In function `main':
/home/ubuntuLove/Documents/workspace_eclipse/SceneRec2/Debug/../src/AdaMain.cpp:5: undefined reference to `AdaBoost<double>::AdaBoost()'
/home/ubuntuLove/Documents/workspace_eclipse/SceneRec2/Debug/../src/AdaMain.cpp:6: undefined reference to `AdaBoost<double>::readFromFile(char*)'
/home/ubuntuLove/Documents/workspace_eclipse/SceneRec2/Debug/../src/AdaMain.cpp:8: undefined reference to `AdaBoost<double>::~AdaBoost()'
/home/ubuntuLove/Documents/workspace_eclipse/SceneRec2/Debug/../src/AdaMain.cpp:8: undefined reference to `AdaBoost<double>::~AdaBoost()'
collect2: ld returned 1 exit status
make: *** [SceneRec2] Error 1
**** Build Finished ****
Note-1. I receive the same result when I execute g++ on terminal.
Note-2. The path of .o files in the argument for linker should be correct (./src/###.o).
AdaBoost.h
#ifndef _ADABOOST_H
#define _ADABOOST_H
#include <iostream>
const double eps = 2.2204e-16;
template<class T>
class AdaBoost
{
int N; //Number of Instances
int D; //Number of Dimensions
int nL; //Number of Learners / Classifiers / Rules
T** fVectors;
int* labels;
void learnRule(int t, double* dist);
double genRule(int t, int* L, double* dist);
public:
//Default Constructor
AdaBoost();
//Constructor
AdaBoost(T** data, int* labels, int n, int d, int nL);
//Train function
void train();
//Test function
void test(double** data, double* pMap);
void test(double** data, double* pMap, int n);
int writeToFile(char* fName);
int readFromFile(char* fName);
//Destructor
~AdaBoost();
};
#endif
AdaBoost.cpp
#include "AdaBoost.h"
#include <fstream>
using namespace std;
template class AdaBoost<double> ;
template<class T>
int AdaBoost<T>::readFromFile(char* fName) {
ifstream inFile;
int temp;
int d, dir;
float thr, wt;
inFile.open(fName);
if (!inFile)
return 0;
inFile >> temp;
this->nL = temp;
int k = 0;
while (!inFile.eof() && k < nL) {
inFile >> d;
inFile >> thr;
inFile >> dir;
inFile >> wt;
k++;
}
inFile.close();
return 1;
}
AdaMain.cpp
#include "AdaBoost.h"
using namespace std;
int main(int argc, char** argv)
{
AdaBoost<double> rdClass;
rdClass.readFromFile("Naerer");
return 0;
}

Answer 1

If you are using explicit instantiation, you have to define the generic version of the member function before instantiating the class:
template<class T>
int AdaBoost<T>::readFromFile(char* fName) {
// ...
}
template class AdaBoost<double>;
However, if you don't have a specific or pressing reason to use explicit instantiation in the first place, go with the other recommendations and define the templates in the header.

Answer 2

You cannot separate template class definition and implementation in different compilation units. In other words, AdaBoost<T> complete implementation should be linked in the same compilation unit that main (where it is used) is.
This is typically fixed by either #including the .cpp file at the end of your .hpp file (if you want to maintain them separate), or just using only the .hpp file implementing the whole class there.

Answer 3

You have multiple problems.
First, you use the unconventional technique of explicit instantiation in a CPP file. As others have pointed out, convention (but nothing more) requires that you put the implementation in the .H file to allow for generic instantiation. You don't have to do this, but if you did, the readfile() error would go away. (As an alternative, put your AdaBoost<double> instantiation after the definition of AdaBoost::readfile.)
Next, you have declared, but not defined, your constructor and destructor. If you wish to use the compiler-provided constructor and destructor you should delete the declarations. If you wish to use your own constructor and dstructor, you should define them.
Best practice is to get rid of AdaBoost.cpp, and modify AdaBoost.h to include the implementation inline in the header. (Note that this best practice is for templated classes; other advice may apply to non-templated classes.)

Answer 4

You need to put the definition of template<class T>
int AdaBoost<T>::readFromFile(char* fName) into AdaBoost.h, and remove AdaBoost.cpp from your build.
It's best to put all template code into the header. C++ linkers are required to eliminate duplicate instantiations of template code so you won't get "multiply defined symbol" errors.
P.S. You should declare the function as template<class T>
int AdaBoost<T>::readFromFile(const char* fName) to get rid of the deprecated conversion from string constant to ‘char*’ warning. The function does not need to alter the filename.