Pixels move when initialized on their own, but not in loop? - c++

Question

I'm working with OpenGL/Glut and trying to make a simple "rain" effect. I have a class for raindrops:
class Drop {
private:
int speed;
int posY;
int posX;
public:
Drop() { // constructor
speed = 5;
posY = 15;
posX = (rand() % 500);
}
void move() {
speed += 1;
posY += speed;
}
int getPosY() {
return posY;
}
int getPosX() {
return posX;
}
};
A list of Drops alongside a function to add Drops to the list.
list<Drop> listDrops;
void placeDrop() {
Drop d = Drop();
listDrops.push_back(d);
}
A function to redraw the OpenGL window:
void refreshDisplay() {
if (rand() % 5 == 1) {
placeDrop();
}
glClear(GL_COLOR_BUFFER_BIT);
glPointSize(5);
glBegin(GL_POINTS);
for (Drop a : listDrops) {
glColor3f(255,255,255); // white
glVertex2i(a.getPosX(),a.getPosY());
a.move();
}
glEnd();
glFlush();
glutSwapBuffers();
}
void repeater(int val) {
glutPostRedisplay();
glutTimerFunc(5, repeater, 0);
}
And of course my main and Glut init:
int main(int argc, char** argv) {
srand(time(NULL));
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE);
glutInitWindowSize(500, 500);
glutCreateWindow("Rain");
gluOrtho2D(0.0, 500.0, 500.0, 0.0);
glutDisplayFunc(refreshDisplay);
glutTimerFunc(0, repeater, 0);
glutMainLoop();
return 0;
}
All of the Drops are initialized properly, I can confirm by outputting their co-ordinates after creation. But they don't move at all. It's like Drop::move() is being ignored.
If I instead initialize a global Drop aDrop (just one), and get rid of the iterator over listDrops in refreshDisplay() and instead just have aDrop drawn, it does move as expected.

Answer 1

You are calling move() on a copy of each Drop that is only scoped for one loop iteration
You can use
//---------
// |
// v
for (Drop &a : listDrops) {
glColor3f(255,255,255); // white
glVertex2i(a.getPosX(),a.getPosY());
a.move();
}
To take the Drops by reference
What you did previously was equivalent to
for (int i=0; i<listDrops.size();++i) {
Drop a = listDrops[i]; //a is a copy of the entry
assert(&a != &listDrops[i])
//...
}