数学向量类通过组合

Question

I started to write a class which would act much like the std::vector but with some mathematical operations. Mainly things like the norm of the vector and overloading important mathematical operators (+,- etc.) which will add, subtract things element wise.

The class is posted below, I have used boost::operators to write all the mathematical operators and they all work flawlessly. I have run into some problems when implementing the iterator. I have tried to write the iterator as a nested class and use boost::iterator to get most/all of the functionality of the std::vector iterator.

This is where I have ran into trouble, the code will not compile with approximately 2 miles worth of template related error output. Which I can post if you are interested but is typical verbose boost template errors.

My question is two-fold.

First is composition the best design choice? Might I do better with private inheritence or a decorator pattern? Or maybe someone knows of an implementation of this idea in a library?

Second, what am I doing wrong with the iterator? I feel as if I am missing something fundamental in my boost::iterator implementation and would like to fix it as opposed to change my design.

I have not included the implementation in most of the methods as they are either trivial or unimportant.

//publicly inherits important boost::operators classes
template <class T>
class Coords: boost::arithmetic<Coords<T>
            ,boost::arithmetic<Coords<T>, T
//              ,boost::indexable<Coords<T>,int,T&
//              ,boost::dereferenceable<Coords<T>, T*>
//            >
    >
>
{
private:
    //private member
    std::vector<T> x_;
public:

    //public constructors
    Coords(int n, T x): x_(n,x){};
    Coords(int n): x_(n){};
    Coords(std::vector<T> &x);
    Coords(const Coords &rhs);

    //iterator nested class, public inherits boost::iterator_facade
    class iterator: public boost::iterator_facade<iterator, Coords<T>, std::random_access_iterator_tag>{
        private:
            typename std::vector<T>::iterator iter_;

            friend class boost::iterator_core_access;

            void increment() { iter_ = iter_++;};

            void decrement() { iter_ = iter_--;};

            void advance(int n){ iter_ = iter_+=n;};

            void equal(iterator const &other) const{
                return this->iter_ == other.iter_;
            }

            T& dereference() const {return *iter_;};

            int distance_to(iterator const &other) const{
                return this->iter_ - other.iter_;
            }

        public:
            iterator():iter_(0){};

            explicit iterator(const typename Coords<T>::iterator& it):iter_(it.iter_){};

            explicit iterator(const typename std::vector<T>::iterator& it):iter_(it){};

    };

    //methods from std::vector I would like to 'copy'
    typename Coords<T>::iterator begin(){return iterator(x_.begin());};
    typename Coords<T>::iterator end(){return iterator(x_.end());};
    typename Coords<T>::iterator operator[](std::size_t n);
    std::size_t size(){return x.size()};

    //mathematical methods
    T norm() const;
    T square() const;
    T sum() const;
    T dotProd(const Coords &rhs);

    //important operator overloads
    Coords operator+=(const T &rhs);
    Coords operator-=(const T &rhs);
    Coords operator*=(const T &rhs);
    Coords operator/=(const T &rhs);
    Coords operator+=(const Coords<T> &rhs);
    Coords operator-=(const Coords<T> &rhs);
    Coords operator*=(const Coords<T> &rhs);
    Coords operator/=(const Coords<T> &rhs);
};

Answer 1

This fixes a number of problems with the program:

#include <boost/operators.hpp>
#include <boost/iterator.hpp>
#include <boost/iterator/iterator_facade.hpp>
#include <vector>
//publicly inherits important boost::operators classes
template <class T>
class Coords: boost::arithmetic<Coords<T>
            ,boost::arithmetic<Coords<T>, T
//              ,boost::indexable<Coords<T>,int,T&
//              ,boost::dereferenceable<Coords<T>, T*>
//            >
    >
>
{
private:
    //private member
    std::vector<T> x_;
public:

    //public constructors
    Coords(int n, T x): x_(n,x){};
    Coords(int n): x_(n){};
    Coords(std::vector<T> &x);
    Coords(const Coords &rhs);

    //iterator nested class, public inherits boost::iterator_facade
    class iterator: public boost::iterator_facade<iterator, T, boost::random_access_traversal_tag >{
        private:
            typename std::vector<T>::iterator iter_;

            friend class boost::iterator_core_access;

            void increment() { ++iter_;}

            void decrement() { --iter_; }

            void advance(int n){ iter_+=n;};

            bool equal(iterator const &other) const{
                return this->iter_ == other.iter_;
            }

            T& dereference() const {return *iter_;};

            int distance_to(iterator const &other) const{
                return this->iter_ - other.iter_;
            }

        public:
            iterator():iter_(0){};

            iterator(const iterator& it):iter_(it.iter_){};
            iterator(iterator& it):iter_(it.iter_){};

            explicit iterator(const typename std::vector<T>::iterator& it):iter_(it){};

    };

    //methods from std::vector I would like to 'copy'
    iterator begin(){return iterator(x_.begin());};
    iterator end(){return iterator(x_.end());};
    iterator operator[](std::size_t n);
    std::size_t size(){return x_.size();};

    //mathematical methods
    T norm() const;
    T square() const;
    T sum() const;
    T dotProd(const Coords &rhs);

    //important operator overloads
    Coords operator+=(const T &rhs);
    Coords operator-=(const T &rhs);
    Coords operator*=(const T &rhs);
    Coords operator/=(const T &rhs);
    Coords operator+=(const Coords<T> &rhs);
    Coords operator-=(const Coords<T> &rhs);
    Coords operator*=(const Coords<T> &rhs);
    Coords operator/=(const Coords<T> &rhs);
};

int main() {
  Coords<int> c(3);
  for(Coords<int>::iterator it(c.begin()); it != c.end(); ++it)
    *it;
}

• The Boost documentation seems to say that the third template parameter to iterator_facade is a boost:: tag, not an std:: tag.
• The second template parameter to iterator_facade is the value type, not the container type.
• The code for increment , decrement , and advance all produced (I think ) undefined behavior.
• When referring to class members from inside the class definition, you don't need to list the class name. There were several places where Coords<T>:: had to be deleted.