Question

I have some trouble with class template inheritance and operators (operator +),
please have a look at these lines:

Base vector class (TVector.h):

template<class Real, int Size>
class TVector {
protected:
    Real* values;
public:

    ...

    virtual TVector<Real, Size>& operator=(const TVector<Real, Size>& rTVector) { //WORKS
        int i = 0;
        while (i<Size) {
            *(values+i) = *(rTVector.values+i);
            i++;
        }
        return *this;
    }
    virtual TVector<Real, Size> operator+(const TVector<Real, Size>& rTVector) const {
        int i = 0;
        Real* mem = (Real*)malloc(sizeof(Real)*Size);
        memcpy(mem, values, Size);
        while (i<Size) {
            *(mem+i) += *(rTVector.values+i);
            i++;
        }
        TVector<Real, Size> result = TVector<Real, Size>(mem);
        free(mem);
        return result;
    }
};

2D vector class (TVector2.h):

template<class Real>
class TVector2: public TVector<Real, 2> {
public:

    ...

    TVector2& operator=(const TVector2<Real>& rTVector) { //WORKS
        return (TVector2&)(TVector<Real, 2>::operator=(rTVector));
    }
    TVector2 operator+(TVector2<Real>& rTVector) const { //ERROR
        return (TVector2<Real>)(TVector<Real, 2>::operator+(rTVector));
    }
};

Test (main.cpp):

int main(int argc, char** argv) {
    TVector2<int> v = TVector2<int>();
    v[0]=0;
    v[1]=1;
    TVector2<int> v1 = TVector2<int>();
    v1.X() = 10;
    v1.Y() = 15;
    v = v + v1; //ERROR ON + OPERATOR
    return 0;
}

Compilation error (VS2010):

Error 2 error C2440: ‘cast de type’ : cannot convert from
‘TVector<Real,Size>’ to ‘TVector2’ …

What is wrong here ? is there a way to do this kind of stuff ?
Just looking for a way to not redefine all my Vectors classes.
I keep searching to do it, but I will be glad to get some help from you guys.

Sorry for bad English,
Best regards.

Answer 1

#include <memory>

using namespace std;

template<class Real, int Size> class TVector {
protected:
  Real *_values;
public:
  TVector() {
    // allocate buffer
    _values = new Real[Size];
  }
  TVector(Real *prValues) {
    // check first
    if (prValues == 0)
      throw std::exception("prValues is null");
    // allocate buffer
    _values = new Real[Size];
    // initialize buffer with values
    for (unsigned int i(0U) ; i < Size ; ++i)
      _values[i] = prValues[i];
  }
  // Do not forget copy ctor
  TVector(TVector<Real, Size> const &rTVector) {
    // allocate buffer
    _values = new Real[Size];
    // initialize with other vector
    *this = rTVector;
  }
  virtual ~TVector() {
    delete [] _values;
  }
  virtual Real &operator[](int iIndex) {
    // check for requested index
    if (iIndex < 0 || iIndex >= Size)
      throw std::exception("requested index is out of bounds");
    // index is correct. Return value
    return *(_values+iIndex);
  }
  virtual TVector<Real, Size> &operator=(TVector<Real, Size> const &rTVector) {
    // just copying values
    for (unsigned int i(0U) ; i < Size ; ++i)
      _values[i] = rTVector._values[i];
    return *this;
  }
  virtual TVector<Real, Size> &operator+=(TVector<Real, Size> const &rTVector) {
    for (unsigned int i(0U) ; i < Size ; ++i)
      _values[i] += rTVector._values[i];
    return *this;
  }
  virtual TVector<Real, Size> operator+(TVector<Real, Size> const &rTVector) {
    TVector<Real, Size> tempVector(this->_values);
    tempVector += rTVector;
    return tempVector;
  }
};

template<class Real> class TVector2: public TVector<Real, 2> {
public:
  TVector2() {};
  TVector2(Real *prValues): TVector(prValues) {}
  TVector2 &operator=(TVector2<Real> const &rTVector) {
    return static_cast<TVector2 &>(TVector<Real, 2>::operator=(rTVector));
  }
  TVector2 &operator+=(TVector2<Real> const &rTVector) {
    return static_cast<TVector2 &>(TVector<Real, 2>::operator+=(rTVector));
  }
  TVector2 operator+(TVector2<Real> const &rTVector) {
    return static_cast<TVector2 &>(TVector<Real, 2>::operator+(rTVector));
  }
  Real &X() { return _values[0]; }
  Real &Y() { return _values[1]; }
};

int main(int argc, char** argv) {
  TVector2<int> v = TVector2<int>();
  v[0]=0;
  v[1]=1;
  TVector2<int> v1 = TVector2<int>();
  v1.X() = 10;
  v1.Y() = 15;
  v = v1;
  v += v1;
  v = v + v1;
  return 0;
}

Some misc notes:

1. it’s very bad that you use malloc against of new. Real can be POD only to allow vector work well in your case. Use new or provide custom creation policy if you think that malloc provides better performance on PODs. Also do not forget to use delete [] instead of free while destroying memory buffer.
2. It’s better to perform bounds checking while overloading operator[]
3. for better performance use ++i instead of postfix form. In former no temporary value is created.