Improve company productivity with a Business Account.Sign Up

x
  • Status: Solved
  • Priority: Medium
  • Security: Public
  • Views: 221
  • Last Modified:

building an array on heap ends in failure (const expected)

When building an array of 2 dimensions on heap it causes an error: const expected and not able for initializing array of dimension (0); source code: struct pixel {char R,G,B} and source code in the application: pixel *pPixel = new pixel[var1][var2];//var1 and var2 are ints.
0
alexott
Asked:
alexott
  • 2
1 Solution
 
inprasCommented:
Hi see here
CPP file
#include <iostream.h>
#include "pixel.h"
void main()
{
      int var1 =10, var2 = 10;
      pixel **pPixel;
      *pPixel = new pixel[var1];//var1 and var2 are ints.
      for (int i = 0; i<=var1; i++)
      {
            pPixel[i] = new pixel[var2];
      }
}
..h file
struct pixel {char R,G,B;
};

Hope this helps
inpras
0
 
ZoppoCommented:
Hi alexott,

this is from MSDN about new operator:

-----------------------
All but the leftmost array dimensions must be constant expressions that evaluate to positive values; the leftmost array dimension can be any expression that evaluates to a positive value.
-----------------------

ZOPPO
0
 
alexottAuthor Commented:
why cant`t you declare an array of datatype[x][y]? Whay does it cause an error ?
0
 
ZoppoCommented:
Hi alexott,

You can declare an array of datatype[x][y] as long as 'y' is of type const.

I think this is because C/C++ does implement an array as follows:

type array[x][y][z][...] is interpreted as an array of x pointers to blocks of size (sizeof(type)*y*z*...). Now, if y, z... are not const, the compiler cannot determine the size of the blocks pointed to, so each time allocating such an array would mean the system has to store the size of the blocks at runtime and with any subscription access use this stored, dynamic size. This would result in a distinct decrease of performance and optimation possibilities.

hope that helps,

ZOPPO
0
Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

Join & Write a Comment

Featured Post

Upgrade your Question Security!

Your question, your audience. Choose who sees your identity—and your question—with question security.

  • 2
Tackle projects and never again get stuck behind a technical roadblock.
Join Now