Changes in class variables not being preserved - c++

Question

This has to be some stupid error, but I have been working at this bug for days and have finally narrowed it down to a bizarre issue. Maybe I'm just not seeing it because most of my experience is programming in java, not C++. I am animating a snowman to move on some terrain in openGL, and whenever I set the velocity of a snowman to be on a random walk, it is not preserved for the next time that the update function is called. I do this by calculating a probability, and if that probability check succeeds, the snowman is given a random velocity and that is added to the position every update. Somehow, the velocities keep getting reset to 0 every time the update function exits, causing them to jump a little bit for one frame, and then teleport back to their previous position for future frames, ultimately not making them move. The x and y position are also restored to their previous value across calls. A sample of the command prompt output is here:
DEBUG: new frame
2) dx dy 0.000000 0.000000
256.000000 15.000000 0.000000 0.000000
2) dx dy 0.000000 0.000000
104.000000 17.000000 0.000000 0.000000
2) dx dy 0.000000 0.000000
256.000000 256.000000 0.000000 0.000000
1) dx dy -0.100000 -0.400000
2) dx dy -0.100000 -0.400000
306.000000 498.000000 -0.100000 -0.400000
2)dx dy 0.000000 0.000000
6.000000 256.000000 0.000000 0.000000
DEBUG: new frame
2) dx dy 0.000000 0.000000
256.000000 15.000000 0.000000 0.000000
2) dx dy 0.000000 0.000000
104.000000 17.000000 0.000000 0.000000
2) dx dy 0.000000 0.000000
256.000000 256.000000 0.000000 0.000000
2) dx dy 0.000000 0.000000
256.000000 256.000000 0.000000 0.000000
2)dx dy 0.000000 0.000000
6.000000 256.000000 0.000000 0.000000
These print out a bunch of values, but the thing to note is that the 4th element should have the same values between calls, and they don't. They get reset.
Snowman.h
#pragma once
#include "ppm_canvas.h"
class Snowman
{
public:
Snowman(canvas_t);
~Snowman(void);
void setWireframe(bool);
void toggleWireframe(void);
void setAnimate(bool);
void toggleAnimate(void);
void setArmSegments(int);
void addArmSegment(void);
void subtractArmSegment(void);
void update(canvas_t);
void draw(void);
private:
bool wireFrame;
bool animate;
bool walking;
int armSegments;
int animationFrameNumber;
float manualUserOffset;
float x, y, z;
float dx, dy;
inline float f(void);
inline void drawMouth(int headRadius);
inline void drawFace(int headRadius);
void drawArm(int remainingSegments);
inline void drawBody();
inline float getHeight(canvas_t);
};
From Snowman.cpp
void Snowman::update(canvas_t texture){
//randomly toggle the walking variable
int probability = rand() % 100;
if(probability <= 2){
walking = !walking;
this->dx = static_cast<float>(( (rand() % 100) - 50))/10.0;
this->dy = static_cast<float>(( (rand() % 100) - 50))/10.0;
printf("1) dx dy %f %f\n", dx, dy);
}
printf("2) dx dy %f %f\n", dx, dy);
//code to control movement
if(walking){
this->animate = true;
this->x += this->dx;
this->y += this->dy;
constrain(this->x, 0, texture.width * 2);
constrain(this->y, 0, texture.height * 2);
}else{
animate = false;
}
//set the height after x and y are resolved
this->z = getHeight(texture);
}
void Snowman::draw(void){
glPushMatrix();{
//turns out x is correct, y is vertical, and z is the other horozintal. oops...
float alittlebit = 10.0;//offset because the center of the bottom sphere is below 0
glRotatef(atan2(dy, dx), 0, 1, 0);
glTranslatef(x, z + alittlebit, y);
drawBody();
}glPopMatrix();
printf(" %f %f %f %f\n", this->x, this->y, this->dx, this->dy );
}
From Main.cpp
inline void drawSnowmen(){
printf("DEBUG: new frame\n");
glPushMatrix();{
glScalef(0.4, 0.4, 0.4);
glBindTexture(GL_TEXTURE_2D, 0);
for(Snowman i:snowmen){
i.update(terrain);
i.draw();
}
glBindTexture(GL_TEXTURE_2D, 1);
}glPopMatrix();
}
I've also included a video of the bug in action below. I'm so desperate to fix this having already spent over 8 hours on this alone, any help would be greatly appreciated.
http://www.youtube.com/watch?v=76uuu1sage0&feature=youtu.be
Also, there is no way to condense the problem to only this, but here is the entire source code:
http://people.ucsc.edu/~cchilder/openGLBug.zip

Answer 1

Try using a ref to the vector element in drawSnowmen(). See my comments above regarding the copy constructor.
for(Snowman &i:snowmen){
i.update(terrain);
i.draw();
}
Without the ref, the update() is modifying a temporary instance of Snowman, not the instance stored in the vector:
for(Snowman i:snowmen){
i.update(terrain);
i.draw();
}