C ++: partial template specialization

Question

I do not get partial pattern specialization. My class is as follows:

template<typename tVector, int A>
class DaubechiesWavelet : public AbstractWavelet<tVector> { // line 14
public:
  static inline const tVector waveletCoeff() {
    tVector result( 2*A );
    tVector sc = scalingCoeff();

    for(int i = 0; i < 2*A; ++i) {
      result(i) = pow(-1, i) * sc(2*A - 1 - i);
    }
    return result;
  }

  static inline const tVector& scalingCoeff();
};

template<typename tVector>
inline const tVector& DaubechiesWavelet<tVector, 1>::scalingCoeff() { // line 30
  return tVector({ 1, 1 });
}

Gcc error outputs:

line 30: error: invalid use of incomplete type ‘class numerics::wavelets::DaubechiesWavelet<tVector, 1>’
line 14: error: declaration of ‘class numerics::wavelets::DaubechiesWavelet<tVector, 1>’

I tried several solutions but no one worked. Does anyone have a clue for me?

Answer 1

template<typename tVector>
inline const tVector& DaubechiesWavelet<tVector, 1>::scalingCoeff() { // line 30
  return tVector({ 1, 1 });
}

This is the definition of a member of a partial specialization, which will be defined as follows

template<typename tVector>
class DaubechiesWavelet<tVector, 1> {
  /* ... */
  const tVector& scalingCoeff();
  /* ... */
};

This is not a specialization of the scalingCoeff member of the main DaubechiesWavelet template. This specialization is required to pass the value of all arguments that your specialization does not. You can use overload to do what you want, though

template<typename tVector, int A>
class DaubechiesWavelet : public AbstractWavelet<tVector> { // line 14
  template<typename T, int I> struct Params { };

public:
  static inline const tVector waveletCoeff() {
    tVector result( 2*A );
    tVector sc = scalingCoeff();

    for(int i = 0; i < 2*A; ++i) {
      result(i) = pow(-1, i) * sc(2*A - 1 - i);
    }
    return result;
  }

  static inline const tVector& scalingCoeff() {
    return scalingCoeffImpl(Params<tVector, A>());
  }

private:
  template<typename tVector1, int A1>
  static inline const tVector& scalingCoeffImpl(Params<tVector1, A1>) {
    /* generic impl ... */
  }

  template<typename tVector1>
  static inline const tVector& scalingCoeffImpl(Params<tVector1, 1>) {
    return tVector({ 1, 1 });
  }
};

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Answer 2

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