class, 2d vectors and bit map

[shotjase]

hey guys i have to implement and test a header file here bitmap..implementation uses a 2-dimensional vector of bool called grid and its dimensions, numrows and numcols. A zero is stored as false in the grid and a one is stored as true. The member functions should manipulate grid, numrows and numcols appropriately.
Look below for more details...

#ifndef BITMAP
#define BITMAP

#include <vector>
#include <iostream>
#include <fstream>
using namespace std;
// class for black/white bitmaps

class bitmap
{
  public:
    bitmap();                    // default constructor 
                                 // generates a 1 cell grid of "off" (0)
    void  display()      const;  // post : the grid has been displayed as stars (for filled cells) and
                                 // spaces (for unfilled cells)
    int   getRows()      const;  // post : returns the number of rows
    int   getCols()      const;  // post : returns the number of columns
    void save (string filename) const; 
                                 // post : the image has been saved in filename
    void read (string filename); // post : the image has been read in from filename
                                 // displays an error message if file is not found
    void invert();               // post : image has been inverted (filled cells
                                 //        changed to empty and vice versa)
                                 
         
    void magnify();              // post : image has been doubled vertically
                                 //        and doubled horizontally
    void rotate();               // post : image has been rotated 90 degrees clockwise
                                 // 
                    
  private:
    int numrows; 
    int numcols;
    vector < vector <bool> >  grid;   // the 2-dimensional grid of data
};
#endif

i now have to write an implemtation file for 

bitmap::bitmap()
{
     // // default constructor 
     // generates a 1 cell grid of "off" (0)        
}

void bitmap::read (string filename)
{
        // post : the image has been read in from filename
       // displays an error message if file is not found

}

void bitmap::display()
{
     // post : the grid has been displayed as stars (for filled cells) 
}

can any tell me what code is meant to go inside these constructors...i am just starting on c++ and am not very good at it..thanks