How c++ subscribes float64 message published from rosserial

edit

Hi, I want to subscribe some data which is published from rosserial_arduino in cpp

I coded like this in sketch

std_msgs::Float64 vx_msg; 

ros::Publisher vx_pub("vx_msg", &vx_msg);

void setup() {

  nh.getHardware() -> setBaud(57600); 

  nh.initNode();

  nh.advertise(vx_pub);

}

void loop() {

  nh.spinOnce();

  vx_msg.data = 1; // publish linear velocity

  vx_pub.publish (&vx_msg);

}

This is for calculating odometry . As I can't use encoder, I have to publish constant velocity which I compute.

I can check that the message is published well because I can watch the data with ' $ rostopic echo'

so I coded with global function in .cpp file

##Like this

double vel_x;

void vx_odom (const vx_pub::vx_msg &vx_msg)

{

    vel_x = vx_msg.vx.msg.data;

}

There are tutorials about how to subscribe string message . But, I don't have idea if It is Float64 type.

What is the form of subsribing float 64 type data ?