Re: Can we override [][] ?

From:
"Cy Edmunds" <spamless.cedmunds@rochester.rr.com>
Newsgroups:
comp.lang.c++
Date:
Wed, 31 May 2006 01:42:14 GMT
Message-ID:
<Wv6fg.9779$8G3.7065@twister.nyroc.rr.com>
"Jim Langston" <tazmaster@rocketmail.com> wrote in message
news:KB5fg.137$212.111@fe04.lga...

I wanted to do an operator override for [][] but couldnt' figure out the
syntax. I tried this (code that doesn't compile commented out with //:

class CMyBitmap
{
public:
   CMyBitmap( int Rows, int Columns ): Rows_( Rows ), Columns_( Columns )
   {
       Data_ = new SPixel[ Rows * Columns ];
   }
   ~CMyBitmap()
   {
       delete[] Data_;
   }

// error C2804: binary 'operator [' has too many parameters
// SPixel& operator[]( const int Row, const int Column )
// {
// return Data_[ Columns_ * Row + Column ];
// }

// error C2092: '[]' array element type cannot be function
// SPixel& operator[][]( const int Row, const int Column )

   SPixel& Pixel( const int Row, const int Column )
   {
       return Data_[ Columns_ * Row + Column ];
   }

private:
   SPixel* Data_;
   int Rows_;
   int Columns_;

   // No copy or assignment yet so disable by making private.
   CMyBitmap ( CMyBitmap const& ) {};
   CMyBitmap& operator=( CMyBitmap const& ) {};

};

Can we override 2d array access?


You should declare your operator as

something operator [] (int Row);

Then "something" must define

SPixel &operator [] (int Column);

The question is, what is "something"? The best thing is probably to make it
a separate class. However, a well known cheat is to make "something" an
SPixel*, which automatically provides this second operator by the usual
rules for subscripting a pointer. Hence

SPixel *operator [] (int Row) {return Row * Columns_;}

will do it!

Cy

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