Re: Array using STL

From:

"Robbie Hatley" <see.my.signature@for.my.email.address>

Newsgroups:

comp.lang.c++

Date:

Thu, 14 Jun 2007 04:22:14 GMT

Message-ID:

<Wn3ci.91761$d22.18988@fe03.news.easynews.com>

"hlg" <huw@ga110n7744.freeserve.co.uk> wrote in message news:4670ad29$0$16397$88260bb3@free.teranews.com...

I have a question, which must surely have occurred to many programmers
since STL first appeared, and yet I have found no reference to it
anywhere, suggesting the problem is insoluble.

Nah. Just means you haven't seen a reference to it.

I wish to create an two-dimensional array of objects. On the one hand,
it is useful to use STL containers for the rows and columns of the
array. On the other, it would be nice to address the elements by the
array element operator [] i.e. myObj obj = myObjArray[x,y];

That is non-standard nomenclature. Array elements in most
languages are Blat[7][5], not Blat[5,7]. I'm not sure your
desired syntax CAN be implimented in C++. You could easily
do Blat(5,7) using an application operator; but Blat[5,7] is
more problematic.

So far I have tried implementing this as:

//
class myObj {...};

//
typedef std::vector<myObj> myObjColumn;
typedef std::vector<myObjColumn> myObjRow;

Ewww. No. A row should be a vector of the objects you want
to store. The 2-D array itself is a (vertical) array of rows.
A "column" would be a slice, better done with valarray.
If you don't needs slices, then you can just use a vector of
vectors:

std::vector<std::vector<MyType> > MyArray;

Or better yet, use a template. Something like the following
(untested, off the top of my head) should work:

template<typename T>
class MyArrayType
{
   public:
      // Construct array:
      MyArrayType(int width, int height)
      {
         repre_ = new std::vector<std::vector<T>* > (height);
         for (int i = 0; i < height; ++i)
         {
            repre_[i] = new std::vector<T> (width);
         }
      }

      // Destruct array:
      ~MyArrayType(int width, int height)
      {
         for (int i = height-1; i >= height; --i)
         {
            delete repre_[i];
         }
         delete repre_;
      }

      // Get reference to element of array:
      T & operator()(int x, int y)
      {
         return representation_[y][x];
      }
   private:
      // Underlying representation:
      std::vector<std::vector<T>* >* repre_;
};

(The above is not guaranteed to be error free. You'll
have to debug and refine it yourself. But it should
give you an idea of a way to proceed.)

myObjRow myObjArray;

//
// retrieve an instance of myObj from array
//
myObj& GetMyObj (UINT x, UINT y)
{
     // check for invalid indexes
     ...
     myObjColumn col = myObjArray[x];
     myObj obj = col[y];
     return obj;
}

So far, so good. Unfortunately, the ratting fratting MS Visual C++
compiler will not allow the next logical step:

#define myObjArray[x,y] GetMyObj(x,y)

... as I get the error message

"error C2008: '[' : unexpected in macro definition"

That's not a Microsoft issue. That's just the way the C
preprocessor is. Functionlike macros use parentheses, not
brackets.

So, has anyone a solution ? (Or am I on the wrong track entirely ?)

If you can use parentheses, Blat(5, 7), then yes. (see above.)

But if you insist on brackets, then I don't see a way to do that.
Maybe someone else here can see a way.

--
Cheers,
Robbie Hatley
lone wolf aatt well dott com
triple-dubya dott Tustin Free Zone dott org