Problem in C++ function template instantiation from class template

main.cpp

#include "myclass.hh"

int main() {
	MyClass<double, 1> myclass;
	MyClass<double, 2> newclass;

	myclass.function();
	newclass = myclass.template convert<2>();
	newclass.function();

	return 0;
}

Makefile

MAIN = program
OBJECTS = main.o myclass.o

all : $(MAIN)

$(MAIN) : $(OBJECTS)
	g++ -o $@ $(OBJECTS)

%.o : %.cpp
	g++ -c $<

clean:
	rm -rf $(OBJECTS) $(MAIN)

myclass.cpp

#include "myclass.hh"

#include <iostream>

using namespace std;

template <typename Number, int Len>
void MyClass<Number, Len>::function()
{
	cout << "function() in MyClass with Len=" << Len << endl;
}

template <typename Number, int Len>
template <int LenNew>
MyClass<Number, LenNew> MyClass<Number, Len>::convert()
{
	MyClass<Number, LenNew> newclass;
	// make new from old here... not relevant for demo
	return newclass;
}

// instantiate template-class with specific template parameters
template class MyClass<double, 1>;
template class MyClass<double, 2>;

// instantiate template-function from template-class with specific template parameters
// against this error: main.cpp:(.text+0x23): undefined reference to `MyClass<double, 2> MyClass<double, 1>::convert<2>()'
template<> template<> MyClass<double, 2> MyClass<double, 1>::convert<2>();

// but doesn't help! still same error

myclass.hh

template <typename Number, int Len>
class MyClass {
public:
	void function();

	template <int LenNew>
	MyClass<Number, LenNew> convert();

private:
	Number n[Len];
};