Why am I getting "function func has internal linkage but is not defined" on mac [duplicate]

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• Why can templates only be implemented in the header file? 10 answers

In the main.cpp I have

#include "helper.h"
#include <list>
using namespace std;
int main(int argc, const char * argv[]) {

    //switch between list and vector
    list<int> a = {0, 1};
    list<char> b = {'a', 'b'};

    func(a, b);

    return 0;
}

My helper.h looks like

template <class C1, class C2>
void func(C1 &a, C2 &b)

and my helper.cpp looks like

#include <list>
#include "helper.h"

template <class C1, class C2>
void func (C1 &a, C2 &b)
{
    //do stuff
}

Then building it in Terminal on Mac with g++ main.cpp mysort.cpp -o sorting -std=c++11 generates the error "function func has internal linkage but is not defined" and Undefined symbols for architecture x86_64: "void func<std::__1::list<char, std::__1::allocator<char> >, std::__1::list<int, std::__1::allocator<int> >, main::$_0>(std::__1::list<char, std::__1::allocator<char> >&, std::__1::list<int, std::__1::allocator<int> >&, main::$_0)", referenced from: _main in main-202a16.o ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation)