operator overloading c++ / where to put my code?

Question

Today I overloaded the << operator in one of my classes:

#ifndef TERMINALLOG_HH
#define TERMINALLOG_HH

using namespace std;

class Terminallog {
public:

    Terminallog();
    Terminallog(int);
    virtual ~Terminallog();

    template <class T>
    Terminallog &operator<<(const T &v);

private:

};

#endif

As you can see I defined the overloaded operator in my header file and I went on implementing it in my .cc file:

//stripped code

template <class T>
Terminallog &Terminallog::operator<<(const T &v) {
    cout << endl;
    this->indent();
    cout << v;
    return *this;
}

//stripped code

Afterwards I created a main.cpp file using my new class:

#include "inc/terminallog.hh"

int main() {
    Terminallog clog(3);
    clog << "bla";
    clog << "bla";
    return 0;
}

and i went on compilying:

g++ src/terminallog.cc inc/terminallog.hh testmain.cpp -o test -Wall -Werror 
/tmp/cckCmxai.o: In function `main':
testmain.cpp:(.text+0x1ca): undefined reference to `Terminallog& Terminallog::operator<< <char [4]>(char const (&) [4])'
testmain.cpp:(.text+0x1de): undefined reference to `Terminallog& Terminallog::operator<< <char [4]>(char const (&) [4])'
collect2: ld returned 1 exit status

BAM! a stupid linker error and I still have no idea where it comes from. I played around a bit and noticed that putting the implementation of my overloaded operator in my header file solves all problems. Now I am even more confused than before.

Why can't I put the implementation of the overloaded operator in my .cc file? Why is it running smoothly when I put it in my header file?

Confused thanks in advance

ftiaronsem

Answer 1

The compiler must see the implementation to be able to use the template. Usually that means you put it in the header.

Answer 2

It is possible to keep implementation in cpp file, but you need to declare usage of your template for every type you are using it with. Please see Parashift C++ Faq for more detailed explanation.

In your case, you have to write that line somewhere in your cpp file:

template Terminallog &Terminallog::operator<<(const char* &v);