Question

I have been encountering a linker error when attempting to call a class method from my main method. I have searched for an answer but nothing seems applicable to my issue, any help will be much appreciated.

FileSystem.h

#ifndef FILE_SYSTEM_H
#define FILE_SYSTEM_H

#include <fstream>
#include <string>
#include <random>

#include "SortingAlgorithms.h"
#include "SortingAlgorithms.cpp"

using namespace std;

class FileSystem
{
private:
    ifstream inFile;
    ofstream outFile;

    template <class T>
    void fillArrays( T randomArray[], T sortedArray[], T backwardArray[], int size );

public:
    FileSystem();
    ~FileSystem();

    bool openFile(string fileName);
    void writeToFile(string output);
    void createTestFiles(string testFiles[]);

    template <class T>
    void fillArrayFromFile(T list[], int size);
};

#endif

fillArrayFromFile

template <class T>
void FileSystem::fillArrayFromFile(T list[], int size)
{
    int i;

    for(i = 0; i < size; i ++)
    {
        inFile >> list[i];
    }

    inFile.clear();
    inFile.seekg(inFile.beg);
}

pa2.cpp

#include <iostream>
#include <ctime>
#include <string>
#include <iomanip>

#include "TimerSystem.h"
#include "FileSystem.h"

using namespace std;

int main()
{
    FileSystem file;
    int testArray[10];

    file.openFile("test-10-0.txt");
    file.fillArrayFromFile(testArray, 10);
}

Error:

pa2.obj : error LNK2019: unresolved external symbol "public: void __thiscall FileSystem::fillArrayFromFile<int>(int * const,int)" (??$fillArrayFromFile@H@FileSystem@@QAEXQAHH@Z) referenced in function _main
C:\Users\Matt\Google Drive\Programming\Data Structures & Algorithms\PA2-9-15-2012\Debug\PA2.exe : fatal error LNK1120: 1 unresolved externals

Thanks again.

Answer 1

When compiling code that calls a template function, the compiler must be able to see the full implementation of the function. At the point the compiler is building pa2.cpp, the compiler is only looking at FileSystem.h and the definitions within the header file (and other header files included from pa2.cpp, of course). It does not look at the contents of FileSystem.cpp at that point.

To fix this, move the implementation of FileSystem::fillArrayFromFile() to the header file instead of the .cpp file. You can do that one of two ways:

class FileSystem {
public:
    template <class T>
    void fillArrayFromFile(T list[], int size);
};

template <class T>
void FileSystem::fillArrayFromFile(T list[], int size)
{
    // ...
}

or

class FileSystem {
public:
    template <class T>
    void fillArrayFromFile(T list[], int size)
    {
        // ...
    }
};