Learning Allegro with C++ :D :(

Hi, my first visit to the Games Programming thread, I guess it was inevitable.
I have attached an assignment that I have been working on in order to learn OOP style programming, and something is screwing me up. I haven't found the source for 24hrs.
My first question- which has no relation to the problem is, what does clear_bitmap(BITMAP) do?
From what I read and how I understood it is that clear_bitmap() function
clears whatever drawing is on a bitmap(). But my world was thrown into confusion when my teacher uploaded his code online.
He blitted a bitmap(which i call player) onto another bitmap(which i call backbuffer) which was then blitted (well draw_sprite() was used) on to the screen- double buffering which i understand.

Now the player can draw itself. It has its own bitmap, a triangle is drawn on the bitmap, and this player bitmap is blitted to the backbuffer which was blitted to the screen.
Before I saw my lecturers code. I didn't use the clear_bitmap(playerbuff) function.
And because of that it appeared as if 2 individual bitmaps (for 2 players) were on the backbuffer, i checked everywhere to see if I could make the player bitmaps transparent to no avail.

When I looked at my lecturers code I found i only needed one line of code! guess?
clear_bitmap()!
so I did

Code:

playerbuff = create_bitmap(100,200) ;
clear_bitmap(playerbuff);
draw_sprite(....) ;

And that worked! it was as if there were 2 actual triangles on the screen and not 2 individual bitmaps if you get what I mean.
but that confused me.
how is it possible that after creating a bitmap, then clearing that bitmap
I was still able to draw on the cleared bitmap and draw it on the backbuffer?

2.
I will post the code, that has made want to do a Padme Amidala

Code:

#include <allegro.h>								  
#include "Player.h"
#include "Vector2.h"  
#include "Obstacle.h"

#include "GameObjects.h"
 

		GameObjects bullets[8] ;
		
void respondToKeyBoard(Player &,Player &) ;
void updateBullets(Player &, BITMAP *) ;
int oldspace  ;
int spacedown ;
			int count ;
			
void fireBullets(BITMAP *);
volatile int ticks ;
void timer()
{
	ticks++ ;
}

int main() 
{
		int oldticks;
		count = 0 ;
	spacedown = 0 ;
	int x=0, y =0; 
		
		
	if(!allegro_init())
	{	 
	
		install_keyboard() ; 
		install_mouse() ; 
		install_timer();
		
		

	
		 Player playerOne(40.0f,50.0f) ; 
		
		 Player playerTwo(140.0f,50.0f) ; 

		Obstacle SightBlinder(400.0f,100.0f);

		Obstacle DarthVader(40, 350.0f);
		
	
		  
		BITMAP *backbuffer = create_bitmap(640,480) ;
			 
		
			install_int_ex(timer,BPS_TO_TIMER(60)) ;
			 srand(time(0)); 
		if(!set_gfx_mode(GFX_AUTODETECT_WINDOWED, 640,480,0,0))
		{  
			
			
			while(!key[KEY_ESC])
			{
					clear_to_color(backbuffer,makecol(0,0,0)) ;
				   	
				   
				
					
				while(ticks==0)
				{
					rest(1);
				}
				while(ticks>0)
				{
					 oldticks = ticks ;

					 
					 		
				
					ticks-- ; 

				
					if(oldticks<=ticks)
					{
						break ;
					}  
				}
				respondToKeyBoard(playerOne, playerTwo)  ; 
				
				
				  				fireBullets(backbuffer);
								////textprintf(screen,font,320,0,makecol(255,0,0),"COunt: %d", count) ;
				
				playerOne.draw(backbuffer) ;
				playerTwo.draw(backbuffer) ; 
				
				
					SightBlinder.draw(backbuffer,40,30,100,110) ;
				
				 DarthVader.draw(backbuffer,40,30,100,110) ;
			
				
				blit(backbuffer,screen,0,0,0,0,SCREEN_W,SCREEN_H) ; 
				
			}
		}
	}


	return 0 ;
}
END_OF_MAIN() 


void respondToKeyBoard(Player &playerOne, Player &playerTwo) 
{

	if(key[KEY_SPACE])
				{
					oldspace = 0 ; 
					spacedown = 1 ;
					if(!oldspace)
					{
				
						for(int index =0;index<=7;index++)
						{
								
								if(!bullets[index].isObjectAlive())
								{
									count++ ;
									bullets[index].setMode(true) ;
									bullets[index].pos.x = playerTwo.getPosX() ; 
									bullets[index].pos.y = playerTwo.getPosY() ;
									
									
									break ; 
								}	
								else
								{
									count =0 ;
								}

										
										
										
										
									
								
									
						}
					}		
				}
				else
				{
					spacedown = 0 ;
					
				}
				

				
			
				
			    
				if(key[KEY_A])
				{
					playerOne.strafe(-0.3f); 
				}
			
				if(key[KEY_D])
				{
					playerOne.strafe(0.3f); 
				}

				if(key[KEY_W])
				{
					playerOne.walk(-0.3f); 
				}
			
				if(key[KEY_S])
				{
					playerOne.walk(0.3f); 
				}



				
				if(key[KEY_LEFT])
				{
					playerTwo.strafe(-0.3f); 
				}
			
				if(key[KEY_RIGHT])
				{
					playerTwo.strafe(0.3f); 
				}

				if(key[KEY_UP])
				{
					playerTwo.walk(-0.3f); 
				}
			
				if(key[KEY_DOWN])
				{
					playerTwo.walk(0.3f); 
				}
			
				

}

void fireBullets(BITMAP *backbuffer)
{
	for(int i=0;i<8;i++)
	{
				 
			
							
		if(bullets[i].isObjectAlive())
		{
					//	bullets[i].draw(backbuffer);
					
				
						

					circlefill(backbuffer,bullets[i].pos.x,bullets[i].pos.y,3,makecol(255,0,0)) ;

						bullets[i].pos.y -= 1.0f;
							  
						 
		}
			if(bullets[i].getPosY()<0)
			{
				bullets[i].setMode(false) ; 
			}
				
				
					
					 
			
						
	}
				
}

What I want to do is, if the user presses space draw a bullet on the screen.
Another space input draw another bullet.
But the thing is it appears to draw all 8 bullets at once. I tried using random velocities in an effort to separate them, no luck. I used random velocity plus timer
and that doesn't work. Not only does it barely separates the bullets, it slows the 2 players down! which isn't fun .
I have no idea what to do at this stage: I believe there is a bug in my code, so I hope one of you guys has experience with allegro and can help me with this please.

I also want to inquire about the install_int (_ex)() function
does it just call a function like every time seconds specified?
like:

Code:

void timer()
{
      ticks++ ;
}
install_int(timer, 60) ;

what does that do? Does it call the function timer() every 60 seconds, thus incrementing ticks?

wow, i'm a great story teller huh?
Well thanks for any future suggestions, hints and answers.

Attachments of the project.

Remaining files attached.
No need to create sprites! I am using vector graphics.
Thanks for your help!