Strange Templates Error

Did I missed something here?

Code:

template<typename T>class A {

public:
  template<typename T2> A<T2> toA() {
    return A<T2>();
  }
};

In the main function, this code just works fine:

Code:

A<int> a1;
A<float> a2 = a1.toA<float>(); // Works fine! As expected.

I think there is nothing wrong with the code,
but G++ gives me syntax errors:

Code:

template<typename T>class B {

  A<T> a;

public:

  B( void ) { ; }
  B( const A<T> &a ) { this->a = a; }

  template<typename T3> B<T3> toB() {
    return B<T3>( a.toA<T3>() ); // error?

    // ???.cpp: In member function `B<T3> B<T>::toB()':
    // ???.cpp:9999: error: expected primary-expression before '(' token
    // ???.cpp:9999: error: expected primary-expression before '>' token
    // ???.cpp:9999: error: expected primary-expression before ')' token
  }
};

I tried to compile it in VC and it gave me a strange error message:

Code:

B<int> b;
b.toB<double>(); // x:\somewhere\code.cpp(9999) : error C2062: type 'double' unexpected

And then I tried another way:

Code:

template<typename T3> B<T3> toB() {
  A<T3> a1 = this->a.toA<T3>(); // won't work

  A<T> a2 = this->a;
  A<T3> a3 = a2.toA<T3>(); // silly, this way won't work too
}

Thank's in advance.

EDIT: Sorry for my English.. T.T

You basically want to create A's out of certain other types of A's. You can do that. Then, you can create B's out of any suitable type of A.

Code:

template <typename T> class A
{
	public:
		A() {}
		template <typename U> A(const A<U> &a) {}
};

template <typename T> class B
{
	private:
		A<T> a;
	public:
		B(const A<T> &aIn): a(aIn) {}
};

int main()
{
	A<int> a1;
	A<float> a2 = a1;	
	B<double> b(a1);
}

I delegate the actual conversion to the copy constructor. A conversion from U to T must be defined, as shown by the example in int to float and int to double. The same applies to any user defined conversion.

Thank you very much whiteflags!

It's amazing!