Re: Keeping type safety for classes derived from template class adapters and few good practice questions

From:

cbarron3@ix.netcom.com (Carl Barron)

Newsgroups:

comp.lang.c++.moderated

Date:

19 Aug 2006 20:37:09 -0400

Message-ID:

<1hkbqlm.1c2fvw1c9vq4sN%cbarron3@ix.netcom.com>

Shimon Shvartsbroit <shimon1981@gmail.com> wrote:

Hello all,

template <int N, typename T>
class VectorAdapter:public boost::additive<VectorAdapter<N, T> >
{
public:
    typedef VectorAdapter<N, T> ValueType;

    // implementation for operators manipulation
    ValueType& operator+=(const ValueType& rhs)
    {
        for (size_t i = 0; i < N; ++i)
        {
           elements_[i] += rhs.elemens_[i];
        }

        return *this;
    }

protected:
  T elements_[N];
};

By using Boost.Operators I have a non member function defined for
operator + as follows:

template <int N, typename T>
VectorAdapter<N, T> operator+(const VectorAdapter<N, T>& lhs,
                            const VectorAdapter<N, T>& rhs);

One possible solution would be to declare VectorAdapter as:
template <int N, typename T, int ClassIdNumber>

ClassIdNumber - unique number for each class derived from
VectorAdapter.

class Vector4: public VectorAdapter<4, float, 0> {..};
class Color4: public VectorAdapter<4, float, 1> {..};
class Other4: public VectorAdapter<4, float, 2> {..};

I am sure there's a better approach.

   There is, it is commonly refered to ax CRTP and it involves creating
a base class templated on the The derived class as well as what ever
other template args required. You used it with boost::additive. Note
boot::additives template paramenter is the name of its derived class.

Now on to the 'fixing' of vector_adaptor...

template <class Derived,class T,int N>
class vector_adaptor:boost::additive<vector_adaptor<Derived,T,N> >
{
protected:
        T elements[N];
public:
        Derived & operator += (const Derived &x)
        {
                for(int i=0;i!=N;++i)
                        elements[i] += x.elements[i];
                return static_cast<Derived &>(*this);
        }
        Derived & operator -= (const Derived &x)
        {
                for(int i=0;i!=N;++i)
                        elements[i] -= x.elements[i];
                return static_cast<Derived &>(*this);
        }
        T & operator [](int x) {return elements[i];}
        const T & operator [](int x) const {return elements[i];}
};

class Vector4:public vector_adaptor<Vector4,float,4>{};
class Color4:public vector_adaptor<Color4,float,4>{};

now the classes passed to boost::aditive<...> are different and operator
+ wilth different 4 element vectors will not compile.

Not tested with boost, but It appears to solve the problem without
adding any complexity to the hierarchies involved beyond the
vector_adaptor class. It does compile with a simple operators lookalike
on this old compiler...

      [ See http://www.gotw.ca/resources/clcm.htm for info about ]
      [ comp.lang.c++.moderated. First time posters: Do this! ]

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