template member definition outside class body [duplicate]

This question already has an answer here:

• Why can templates only be implemented in the header file? 9 answers

I found that I cannot compile my program when I put my class member implementation outside the header. I simplified the code to show the problem more exactly.

I define my class in dog.h,

class Dog {
public:
    template<typename T>
    void bark();
}

I cannot template the class because I know the type of template argument only when I call bark(). The implementation is as follows,

#include "dog.h"
template<typename T>
void Dog::bark() {}

And here comes my main.cpp

#include "dog.h"
int main() {
    Dog d;
    d.bark<int>();
    return 0;
}

I got the following compilation error,

/tmp/ccoxjr6t.o: In function `main':
test.cpp:(.text+0x10): undefined reference to `void Dog::bark<int>()'
collect2: error: ld returned 1 exit status

However everything is fine when I put the implementation to dog.h. Can anyone tell me the reason? The compiler I use is GCC-4.8.1. Thanks!