Question

I’m having some trouble with multi-layer pointers. Basically I’m reading point locations from a file and using them to map out polylines.

I’m trying to create a dynamically allocated data structure that will change depending on the information contained in the file.

Each file is structured like this.

29 // number of polylines in the whole file
3 // first polyline, number of points in it
32 435 // first coordinate where x = 32 and y = 435
15 200
100 355
10  // second polyline, number of points in it
245 35
330 400

and so on etc.

I created a struct with x and y ints to hold the coordinates for each point

struct coordinates{
  int x;
  int y;
};

I want to basically create a data structure like this …

Pointer --> array w/ num of polys
             |      |      |
             |      |      |
             v      v      v
           poly0   poly1  poly2       // arrays with coordinate structs
            x1,y1   x1,y1  x1,y1
            x2,y2   x2,y2  x2,y2
            x3,y3   x3,y3  x3,y3

Here’s what my code looks like

coordinates *** dinoPoints;

struct coordinates{
      int x;
      int y;
 };

void myInit(void){...} // just has initialization stuff for the draw window

void loadDino (char * fileName)
{
  fstream inStream;
  inStream.open(fileName, ios ::in); // open the file
  if(inStream.fail())
  return;

  GLint numpolys, numlines; // these are just regular ints

  inStream >> numpolys; //reads in number of polys

  //dynamically allocates the number of polys in file to datastructure
  dinoPoints = new coordinates**[numpolys]; 

  for(int j = 0; j < numpolys; j++){  // read each polyline

      inStream >> numlines;  // read in number of lines in polyline

      dinoPoints[j] = new coordinates*[numlines];

      for (int i = 0; i < numlines; i++){  // allocate each set of coords
          dinoPoints[j][i] = new coordinates;

          // read in specific point coordinates
          inStream >> dinoPoints[j][i].x >> dinoPoints[j][i].y;
      }
   }        

   inStream.close();
}

void myDisplay(void)
{
    drawDino();   // draws the dinosaur on the screen
}

//still writing this function. Calls myDisplay through glutDisplayFunc()
// and also calls loadDino with filename passed as a parameter

void main(int argc, char **argv){...}

So for some reason it’s giving me “expression must have classtype” errors on the line

inStream >> dinoPoints[ j ][ i ].x >> dinoPoints[ j ][ i ].y;

Also normally the IDE (Visual Studios 2010) will show the different elements of a data structure after a period is typed, but after typing “dinoPoints[ j ][ i ]. ” it doesn’t show up with any contained elements to select from, which means it doesn’t even know what I’m talking about in regards to dinoPoints[ j ][ i ]

Does anyone know what I’m doing wrong? I feel like i’m missing something in regards to how the multilevel pointers work, but I’m not sure exactly what.

Answer 1

You’ve got a triple-layered pointer. You need three de-references in there. You’ve only got two with dinoPoints[j][i]– the result of that expression is a pointer.

Not to mention the horrendous unsafety of what you’re doing. Use a vector<vector<vector<coordinates>>> for this- it’s safer and easier to understand.