Question

Possible Duplicate:
What is an undefined reference/unresolved external symbol error and how do I fix it?

I have a median function that finds a median from elements of the array as follows

median.h

template <class N>
N median (N*,size_t);

median.cpp

#include "median.h"
template <class N> 
 N median (N* numbers,size_t size){
    size_t mid = size/2;
    return size % 2 == 0? (numbers[mid] + numbers[mid-1])/2 : numbers[mid];
}

main

#include <iostream>
#include "median.h"

using namespace std;

int main(){
    double Numbers [] = {1,2,3,4,5,6,7};


    size_t size = sizeof(Numbers)/sizeof(*Numbers);

    double med = median(Numbers,size);
    cout << med << endl;
    return 0;

}

But I get the following error

main.obj : error LNK2019: unresolved external symbol "double __cdecl median<double>(double *,unsigned int)" (??$median@N@@YANPANI@Z) referenced in function _main

Answer 1

Templates should instantiated by compiler before they used. In median.h you have declaration of median template, and in media.cpp you have its implementation but you don’t use it in that file, so template won’t instantiated in that file. On other side on main.cpp you try to instantiate template but there is no implementation, just declaration! So compiler will fail to instantiate template’s implementation and that’s the cause of your error. Just move implementation to median.h or instantiate the template in median.cpp by using:

template double median<double>(double*, size_t);