In class filter.h, I have the following forward declarations

template<class T> //Forward declaration due to circular dependency
class Agc;
class FixPt;
class QuantizerCmplx;
template<class T>
class Quantizer;

I get the following errors while compiling with g++.

../source/filter.h: In member function ‚int FarrowFilter<T>::filter_quantize()‚:
../source/filter.h:1837: error: invalid use of undefined type ‚struct QuantizerCmplx‚
../source/filter.h:12: error: forward declaration of ‚struct QuantizerCmplx‚

How do I solve this problem? It compiles fine with VC++