How to convert Float 64 to Double?

edit

Hi I have the following plugin: I like to subscribe to a ROS node which gives me the reset_frequency data (in Float 64) which by default has a value of 2.0. However when compiling, I receive this error:

Error:

invalid use of member ‘gazebo::ParticleShooterPlugin::reset_frequency’ in static member function
 reset_frequency= msg->data/1.0
/home/ntu-saab_uwr/uuvsim_ws/src/particle_shooter/particle_shooter/src/particle_shooter_plugin_modify.cpp:323:28: note: declared here
   double reset_frequency = 2.0;

Code:

......

void particle_callback(const std_msgs::Float64::ConstPtr& msg)
{
    reset_frequency= msg->data;
}

......

double reset_frequency = 2.0;

May I Know how to solve this Problem?