Subscribing to a Bool msg via rosserial_arduino.

edit

You should use the message types directly. For example:

void myCallback(const std_msgs::Bool& msg);
ros::Subscriber<std_msgs::Bool> mySubscriber("~some_topic", &myCallback);
...
void myCallback(const std_msgs::Bool& msg) {
    if (msg.data) {
        ... do something ...
    } else {
        ... do something else ...
    }
}

See the rosserial_arduino subscriber tutorial. There, they use the Empty message type. The callback parameter type is just const std_msgs::Empty&. All the rosserial_arduino tutorials are worth a look.

A few caveats about rosserial_arduino. It uses quite a bit of memory, not a problem if you are using a Mega or a Teensy, for example, but can be limiting on an Uno or Leonardo. You can limit memory by declaring the node handle in a way slightly different than the tutorials:

ros::NodeHandle_<ArduinoHardware, 5, 14, 125, 125> nh;

Using the NodeHandle_ template type directly allows you to control the buffer sizes and the number of subscribers and publishers in the declaration, rather than having to modify <ros.h>. The parameters 5, 14, 125, and 125 above are, respectively, the number of subscribers you will declare, the number of publishers, the size of the input buffer in bytes, and the size of the output buffer in bytes. Those buffer sizes are very skimpy, because I was running out of memory (leading to my abandonment of rosserial_arduino). You need to have enough buffer size to hold the largest message you will send or receive, and you have to call nh.spinOnce() often enough that the output buffer doesn't fill up.

ROS messages can be large. Also, because the Atmel chip is a non-Von Neumann architecture, static strings also take up RAM, so long topic names will start to eat up precious memory.

As I said, you don't have to worry much about this on a Mega or Teensy. I had a lot of stability issues on a Leonardo-class board which housed my power supply and motor controller. I eventually gave up and used ros_arduino_bridge instead.