How do I make projectiles?

I am totally stumped on this one. I'm us开发者_开发问答ing C++ and SFML 1.6 for a game I'm developing, and I have no bloody idea. How do I make projectiles (like bullets)? I just don't understand it. It could be my lack of sleep but I don't know.

So my question is how do I create a Sprite that moves in a definite direction based on where the mouse is? (Think of a top down shooter with mouse aiming)

Easiest solution: If the mouse is at Mx,My and the ship is at Sx,Sy then calculate the direction from the ship to the mouse: Dx=Sx-Mx Dy=Sy-My

Now normalise D (this means scale it so that it's length is one):

DLen=sqrt(Dx*Dx + Dy*Dy)
Dx/=DLen;
Dy/=DLen;

Now Dx is the distance you want to move the bullet on the x axis in order to get bullet speed of 1.

Thus each frame you move the bullet like so (position of bullet: Bx,By Speed of bullet: Bs [in pixels per millisec] Frame time Ft[in millisec])

Bx=Bx+Dx*Bs*Ft
By=By+Dy*Bs*Ft

This give you a bullet that moves towards the mouse position at a speed independent of the direction of the mouse or framerate of the game.

EDIT: As @MSalters says you need to check for the DLen==0 case when the mouse is directly above the ship to avoid division by zero errors on the normalise

One way to do it is to make the bullet face the mouse and then move it across the x and y axis by using trigonometry to find the hypotinuse from the angle. I don't think i explained this very well, so here the code to make a sprite move from its rotation:

void sMove(sf::Sprite& input,float toMove, int rotation){
bool negY = false;
bool negX = false;
int newRot = rotation;
if (rotation <= 90){
    negY = true;
}
else if (rotation <= 180){
    //newRot -= 90;
    negY = true;
}
else if (rotation <= 360){
    newRot -= 270;
    negY = true;
    negX = true;
}
float y = toMove*cos(newRot*PI/180);
float x = toMove*sin(newRot*PI/180);
if (negY){
    y = y*-1;
}
if (negX){
    x = x*-1
}
input.move(x, y);
}