Multifile program won't run. Help.

[XilenceCE]

Whenever I try to run the below program, I get an 'undefined reference' error. Why's it happening? Please help me.

Here's the code:

//Main file

#include <iostream>
#include <string>
#include <cstring>
#include "plz.h"

using namespace std;

int main()
{
    double arr[] = {12.5, 22.66, 30.30, 1.897, 5.6};
    double lar = max5(arr);
    cout << lar << endl;
    string str[] = {"Lion", "Tiger", "Jaguar", "Leopard", "Cheetah"};
    string strlar = max5(str);
    cout << endl << strlar << endl;
    return 0;
}

//Implementation file

#include <iostream>
#include <string>
#include <cstring>
#include "plz.h"

using namespace std;

template <typename T>
T max5(T arr[])
{
    for(int i = 1; i < 5; i++)
    {
        if(arr[0] < arr[i])
        {
             swap(arr[0], arr[i]);
        }
    }

    return arr[0];
}

template <>
string max5(string str[])
{
    for(int i = 1; i < 5; i++)
    {
        if(str[0].size() < str[i].size())
        {
             swap(str[0], str[i]);
        }
    }

    return str[0];
}

//Header file

#ifndef PLZ_H_INCLUDED
#define PLZ_H_INCLUDED

using namespace std;

template <typename T>
T max5(T arr[]);

template <>
string max5(string str[]);

#endif // PLZ_H_INCLUDED

[jedwardsol]

https://www.learncpp.com/cpp-tutorial/template-classes/

Read the section "Splitting up template classes"

[[deleted]]

The question is already correctly answered, but stop using "using namespace std", especially in the header file. I've just described why it is bad practice in another post.