防止忘记求角度

double len_y = pos.y - start_pos.y;
double len_x = pos.x - start_pos.x;

double tan_yx = tan_yx = abs(len_y)/abs(len_x);
float angle = 0;
if(len_y > 0 && len_x < 0)

 {
    angle = atan(tan_yx)*180/M_PI - 90;
} 

else if (len_y > 0 && len_x > 0) 

{
   angle = 90 - atan(tan_yx)*180/M_PI;
} 

else if(len_y < 0 && len_x < 0)

 {
   angle = -atan(tan_yx)*180/M_PI - 90;
} 

else if(len_y < 0 && len_x > 0) 

{
   angle = atan(tan_yx)*180/M_PI + 90;
}