So I can move my object down and up by changing the
x += velocity;
y += velocity;
SFML(Simple Fast Multimedia Library)
In SFML rotating an object won't affect the direction where it is going. So you have to implement it yourself.
For diagonal moving angle=45 degrees or (45*PI)/180 radians depending which sin/cos library you are using(C/C++ library require radians as argument).