Re: Templating classes

From:

shamino@techie.com (David C.)

Newsgroups:

comp.lang.c++

Date:

01 Jun 2007 18:07:01 -0400

Message-ID:

<ur6ov72ii.fsf@qqqq.invalid>

Also, 'std::vector<T>::iterator' is a dependent name. The compiler
doesn't know that it can be used where a type is expected. You need
to tell the compiler to trust you:

for (typename std::vector<T>::iterator ...
^^^^^^^^

This is one of the reasons I like to use typedefs. It makes the code
easier to read and removes ambiguities. For example

    template<class T> class MatrixBase
    {
      protected:
        typedef std::vector<T> matrixrow_t;
        typedef std::vector<matrixrow_t> matrix_t;

        matrix_t matrix;
          ...

      public:
        void AddRow(const T &d = T())
        {
            for(matrix_t::iterator i = matrix.begin();
                i != matrix.end(); ++i)
            {
                i->push_back(d);
            }
            ++height;
        }
        ...
    };

Doing this would also make another error in the OP's code more obvious:

template<class T> class MatrixBase
{
  public:
    ...
      void AddRow(const T &d = T())
      {
         for (std::vector<T>::iterator i = matrix.begin(); i !=
matrix.end(); ++i)
            i->push_back(d);
         ++height;
      }
      // More methods
   protected:
      std::vector< std::vector<T> > matrix;
      unsigned height;
      unsigned width;
};

matrix.begin() returns an iterator to a std::vector< std::vector<T> >,
not an iterator to a std::vector<T>.

Using typedefs, you'd probably notice that bug without ever even running
it through the compiler.

-- David