Unable to push_back elements in a list - c++

Question

I'm facing a really peculiar problem with my code,
I am unable to push elements into a list.
I'm trying to implement a scan fill algorithm.
I want the list of points that I have plotted on the screen so that I can check if a scan line intersects with them.
In my Screen::plot_pixel function I push the Point into the plotted_points list.
But when I iterate through the list, it's empty.(I'm iterating in shape.cpp using a friend function)
I tried using a set too but to no avail. I have attached the console output I get too.
plot_pixel gets called multiple times, I verified this by adding a print statement there but the points refuse to get pushed in.
Here's all my code, point.h
#ifndef POINT_H
#define POINT_H
class Point
{
float x_coordinate,y_coordinate;
public:
Point(){}
Point(float x, float y):x_coordinate(x),y_coordinate(y){}
float get_x() const{return x_coordinate;}
float get_y() const {return y_coordinate;}
bool operator==(const Point rhs)const
{
if( ((int)x_coordinate == (int)rhs.get_x()) && ((int)y_coordinate == (int)rhs.get_y()) )
return true;
else return false;
}
bool operator<(const Point rhs)const
{
if((int)x_coordinate < (int)rhs.get_x())
return true;
else return false;
}
};
#endif
screen.h
#ifndef SCREEN_H
#define SCREEN_H
#include<graphics.h>
#include "point.h"
#include<list>
class Shape;
class Screen
{
private:
int max_x,max_y,grid_size;
int delta_x,delta_y;
int origin_x,origin_y;
public:
std::list<Point> plotted_points;
// Default Constructor to Initialize the screen with the grid
Screen(int, int, int);
//Method prototypes
void plot_pixel(int,int,int);
void dda_line(Point p1, Point p2);
int get_max_x(){return max_x;}
int get_max_y(){ return max_y;}
friend void draw_shape(Screen,Shape);
friend void fill_shape(Screen,Shape);
};
#endif
screen.cpp
#include "screen.h"
void Screen::plot_pixel(int xcoord,int ycoord,int col=WHITE)
{
int l,t,r,b;
l = origin_x + xcoord * delta_x;
r = l + delta_x;
b = origin_y - ycoord * delta_y;
t = b - delta_y;
setcolor(col);
bar(l,t,r,b);
setcolor(WHITE);
Point *tmp = new Point(xcoord,ycoord);
//the culprit code
plotted_points.push_back(*tmp);
delete tmp;
}
void Screen::dda_line(Point p1, Point p2)
{
.....
plot_pixel(x,y,0);
}
shape.h
#ifndef SHAPE_H
#define SHAPE_H
#include<list>
#include<iostream>
#include "point.h;
class Screen;
class Shape
{
list<Point> figure;
public:
Shape(list<Point> s);
friend void draw_shape(Screen,Shape);
friend void fill_shape(Screen,Shape);
};
#endif
shape.cpp
#include "shape.h"
#include "screen.h"
Shape::Shape(list<Point> s):figure(s){}
void fill_shape(Screen scr, Shape sh)
{
list<Point>::iterator pit,vit;
//HERE's Where I try iterating over the list and get nothing
//to check if the plotted points and vertices are listed correctly
cout<<"Plotted Points :\n";
for( pit = scr.plotted_points.begin();pit != scr.plotted_points.end();pit++)
{
cout<<pit->get_x()<<" "<<pit->get_y()<<endl;
}
cout<<"Vertices :\n";
for( vit = sh.figure.begin();vit != sh.figure.end();vit++)
{
cout<<vit->get_x()<<" "<<vit->get_y()<<endl;
}
}
and finally my driver
#include "screen.h"
#include "shape.h"
using namespace std;
int main()
{
Screen s(641,641,32);
Point p1(-10,-10),p2(-10,10),p3(10,10),p4(10,-10);
list<Point> rectangle_points;
//to construct the rectangle
rectangle_points.push_back(p1);
rectangle_points.push_back(p2);
rectangle_points.push_back(p3);
rectangle_points.push_back(p4);
Shape rectangle(rectangle_points);
draw_shape(s,rectangle);
fill_shape(s,rectangle);
getch();
return 0;
}
Here's the output
Plotted Points :
Vertices :
-10 -10
-10 10
10 10
10 -10

Answer 1

You are passing the Screen object s into your functions, but they take the Screen object by value. They therefore take a copy of the object which they then modify. Your original Screen "s" is unchanged after the call to draw_shape().
You should probably modify them so they take a reference to a Screen object, such as:
void draw_shape(Screen& scr, Shape shp)