Why am I getting an undefined reference error?

[exotic tide]

utility.h

#define __UTILITY_H__

std::string lowerStr(const std::string& str);
template <typename TP> std::time_t to_time_t(const TP tp);

#endif```
**utility.cpp**
```#include <iostream>
#include <string>
#include <set>

#include <chrono>

#include "utility.h"

std::string lowerStr(const std::string& str){
    std::string result = str;
    std::string::iterator itr = result.begin();
    while (itr != result.end()){
        if (!isdigit(*itr) && !isalpha(*itr)){
            itr = result.erase(itr);
        }
        else{
            if (isalpha(*itr)){
                *itr = tolower(*itr);
            }
            itr++;
        }
    }
    return result;
}

template <typename TP>
std::time_t to_time_t(const TP tp){
    using namespace std::chrono;
    auto sctp = time_point_cast<system_clock::duration>
                (tp - TP::clock::now() + system_clock::now());
    return system_clock::to_time_t(sctp);
}```
`main.cpp` not included, but the code calls upon both lowerStr() and to_time_t(). However, when I compile using `g++ main.cpp utility.cpp -o main.exe -g -Wall -Wextra`, I get an undefined reference error only for to_time_t() and can't figure out why. Probably a dumb issue but I would greatly appreciate if someone could clear this up to me.

[random plover]

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[ashen river]

You have a template defined in a .cpp file, common practice is to put them in the header entirely

The compiler needs to see the definition so it can substitute the typed properly

[exotic tide]

template functions should always be implemented in header files?

[ashen river]

Yes

Don't worry about ODR with templates, I believe there's some kind of exemption

[exotic tide]

ty for the clarification!

sorry but whats ODR?

[ashen river]

One definition rule

Every function must only be defined once

[exotic tide]

ah okok

thank you very much lol

!solved