Thread: Undefined Reference to a generic function error

I'm doing this example that involves two generic functions, one that takes a vector of type T and another that takes an array of type T. The functions worked when I put both the declaration and definition within main, but when I split them up between the header file and implementation I get an undefined reference to the functions when I call them from main.

Code:

//main.cpp
#include <cstdlib>
#include <iostream>
#include <vector>
#include "headers.h"



using std::vector;
using std::cout;
using std::endl;


int main()
{

    int a_grades[8] = {87, 95, 100, 83, 75, 49, 56, 94};  
    vector<int> v_grades(a_grades, a_grades + 9);
       
    cout << median(a_grades) << endl;
    cout << median(v_grades) << endl;
    
    //49 56 75 83 87 94 95 100
    
    system("PAUSE");
    return EXIT_SUCCESS;
}

Code:

//headers.h
#ifndef headers_h
#define headers_h

#include <vector>

template <class T> double median(std::vector<T> vec);
template <class T> double median(T vec);

#endif

Code:

//headers.cpp
#include <iostream>
#include <vector>
#include <stdexcept>
#include <cstddef>
#include "headers.h"

using std::vector;


template <class T> double median(vector<T> vec)
{

	typedef typename vector<T>::size_type vec_sz;

	vec_sz size = vec.size();
	if (size == 0)
		throw std::domain_error("median of an empty vector");

	std::sort(vec.begin(), vec.end());

	vec_sz mid = size/2;

	return size % 2 == 0 ? (vec[mid] + vec[mid-1]) / 2 : vec[mid];
}

template <class T> double median(T vec)
{   
    size_t size = sizeof(vec)/sizeof(*vec);
    
	if (size == 0)
		throw std::domain_error("median of an empty vector");

	std::sort(vec, vec + size);
	    
           
	size_t mid = size/2;

	return size % 2 == 0 ? (vec[mid] + vec[mid-1]) / 2 : vec[mid];   
}

errors

Code:

In function `main': 
[Linker error] undefined reference to `double median<int*>(int*)' 
[Linker error] undefined reference to `double median<int>(std::vector<int, std::allocator<int> >)' 
ld returned 1 exit status
[Build Error]  [practice.exe] Error 1

template implementations must be #include'd by the code using them. typically this means defining the template class or functions in a header.

basiaclly move all your code in headers.cpp to headers.h and you'll be sweet. If you're interested why this is google for the C++ export keyword. (only one compiler that I've heard of supports this, Comeau C++ http://www.comeaucomputing.com/tryitout/ )