SFML_Vector

sf::Vector2f ball_current_speed(0,0); //ball starts with no movement, OK

// Lets say the game begun, we launch the ball in a random direction
     // e.g. one program execution would make: ball_current_speed = (9,4)
     // if you remember good' ol math, you can use the pythangorean theorem to find 
     // it's actual speed, which in this case is sqrt(9^2 + 4^2)
ball_current_speed += sf::Vector2f(rand()%10,rand()%10);

//stuff happends

// Now we saw it touched a paddle, so we want to deflect the ball in the opposite
    // direction, meaning we want to reverse it's x & y speed vector. If you think
    // about it this will simulate a paddle deflecting a ball
ball_current_speed.x = -ball_current_speed.x;
ball_current_speed.y = -ball_current_speed.y;

// It's important to realize this is the SPEED vector, not a POSITION vector. The
    // position vector will be manipulated using the speed vector!