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

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.