C++ Dynamic multidimensional array problem

Question

I'm developing a 2d-platformer. Everything was fine until I've got some hard to solve problem. Level map is stored in dynamic multidemension array(char **map). It works fine, until I want to redefine it

Here's the part of code:

Map& Map::operator=(const Map& rhs)
{
    if(width!=0||height!=0)
    {
        for(int i=0;i<width;i++)
            delete[] map[i];
        delete[] map;
    } //deleting previously created array

    height=rhs.height;
    width=rhs.width; //receiving other map's size

    map=new char* [width];
    walkmap=new unsigned char* [width];
    objmap=new char* [width];
    for(int i=0;i<width;i++)
    {
        *(map+i)=new char[height];
    } //creating new array

    for(int h=0;h<height;h++)
        for(int w=0;w<width;w++)
        {
            map[w][h]=rhs.map[w][h];
        } //receiving new values

    //...
}

Everything works fine for the first time, but when I need to redefine array for the second time my program crashes at the part, when array is receiving new values from another one. May be I miss something, but I can't find it! I was searching for this problem, but didn't find what I am doing wrong. Help me, please.

You've left off the declaration of the type(def?) Map, which is rather important to the question. — Ernest Friedman-Hill
– Ernest Friedman-Hill, Commented May 10, 2011 at 13:38
Also I would change *(map+i) to map[i] just because its much more descriptive of what you are trying to accomplish with that line IMHO. — Chad
– Chad, Commented May 10, 2011 at 13:41

Nick From Montreal · Accepted Answer · 2011-05-10 14:46:11Z

1

As always, Boost has an elegant and memory efficient multi-dimensional array class:

http://www.boost.org/doc/libs/1_39_0/libs/multi_array/doc/user.html

For example, to setup a 10 x 20 array of bool values:

 
    boost::multi_array  mtaFlagMatrix(boost::extents[10][20]);

Then to access its elements:

 
    mtaFlagMatrix[2][6] = false; // indexes are zero-based - just like normal C arrays
     ...
    if ( mtaFlagMatrix[2][6] )
    {
       ...
    }

Then, you can resize the array this way (existing values are preserved):

 
typedef boost::multi_array array_type;

array_type::extent_gen extents;
  array_type A(extents[3][3][3]);
  A[0][0][0] = 4;
  A[2][2][2] = 5;
  A.resize(extents[2][3][4]);
  assert(A[0][0][0] == 4);
  // A[2][2][2] is no longer valid.

This saved me a lot of time testing for extreme cases.

answered May 10, 2011 at 14:46

Nick From Montreal

813 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

moarthenfeeling Over a year ago

Oh, I don't want use additional components, because my code'll be hard to undrerstand. I don't need to change the size very often. Just 2 or 3 times! And using libraries won't help me to understand, when I was wrong D:

Joel Falcou · Accepted Answer · 2011-05-10 13:44:04Z

0

Your 2d array is not freeed properly. I advise you to use the Iliffe way of allocating 2d arrays which is faster and safer to use:

char** alloc_array( int h, int w )
{
  typedef char* cptr;
  int i;
  char** m = new cptr[h];
  m[0] = new char[h*w];
  for(i=1;i<h;i++) m[i] = m[i-1]+w;
  return m;
}

void release_array(char** m)
{
  delete[] m[0];
  delete[] m;
}

int main()
{
  int r,c;
  char** tab;
  int width  = 5;
  int height = 3;
  tab = alloc_array(height, width); /* column first */

  for(r = 0;r<height;++r)
   for(c = 0;c<width;++c)
    tab[r][c] = (1+r+c);

  for(r = 0;r<height;++r)
  {
    for(c = 0;c<width;++c)
    {
      printf("%d\t",tab[r][c]);
    }
    puts("");
  }
  release_array(tab);
}

This can be easily encapsulated in a neat 2d-array class or made to use std::vector instead of raw allocation. Prefer this way of doing 2d array as it is cache friendly , style provide the [][] access and is no slower and soemtimes faster than the polynomial 1d access.

answered May 10, 2011 at 13:44

Joel Falcou

6,3871 gold badge19 silver badges35 bronze badges

5 Comments

moarthenfeeling Over a year ago

Thanks, I'll try to use this method

moarthenfeeling Over a year ago

Aww, same problem. It works fine for the first time, then I delete previously created array, create new array(everything is okay now), but when I start to assign some new values to the new array program crashes again, same is in my code.

Joel Falcou Over a year ago

you'r doign something wrong elsewhere. can you show me your updated code usign these functions

moarthenfeeling Over a year ago

pastebin.com/Gtvnf2wA Here it is. (For the first time rhs.tileset!=0, but fir the second it is 0

moarthenfeeling Over a year ago

Oh, and the other arrays use the same method(but it's not shown in the code)

Collectives™ on Stack Overflow

C++ Dynamic multidimensional array problem

2 Answers 2

1 Comment

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related